Soluble proteins of the nacre of the giant oyster Pinctada maxima and of the abalone Haliotis tuberculata: extraction and partial analysis of nacre proteins

Overview of structure, function and integrated utilization of marine shell

Marine shell resources have received great attention from researchers owing to their unique merits such as high hardness, good toughness, corrosion resistance, high adsorption, and bioactivity. Restricted by the level of comprehensive utilization technology, the utilization rate of shells is extremely low, resulting in serious waste and pollution. The research shows that the unique brick-mud structure of shells makes them have diverse and good functional characteristics, which guides them to have great utilization potential in different fields. Hence, this review highlights the constitutive relationship between microstructure-function-application of shells (e.g., gastropods, cephalopods, and amniotes), and the comprehensive applications and development ideas in the fields of biomedicine, adsorption enrichment, pHotocatalysis, marine carbon sink, and environmental deicer. It is worth mentioning that marine shells are currently well developed in three areas: bone repair, health care and medicinal value, and drug carrier, which together promote the progress of biomedical field. In addition, an in-depth summary of the application of marine shells in the adsorption and purification of various impurities such as crude oil, heavy metal ions and dyes at low-cost and high efficiency is presented. Finally, by integrating thoughts and approaches from different applications, we are committed to providing new pathways for the excavation and future high-value of shell resources, clarifying the existing development stages and bottlenecks, promoting the development of related technology industries, and achieving the synergistic win-win situation of economic and environmental benefits.

Extraction, Purification, Bioactivities and Application of Matrix Proteins From Pearl Powder and Nacre Powder: A Review

Natural pearls are formed when sand or parasites (irritants) accidentally enter into the oyster body and form pearls under the cover of the nacre layer. Pearl powder is a powdery substance by grinding pearls into small grains, however, the nacre powder is the inner layer of outer corner layer and middle prism layer. Pearl medicine in China has a history of more than 2,000 years, pearl has the effects of calming the mind, clearing the eyes, detoxifying the muscle and so on. In this paper, the researches on the extraction of pearl powder and nacre powder, the isolation and purification of matrix protein and the various biological activities (osteogenic activity, antioxidant, anti-inflammatory, anti-apoptotic, promoting the migration of fibroblasts, and so on) are reviewed in detail. To provide readers with a faster understanding, the method of extraction and purification and the application of nacre powder and pearl powder are clearly presented in the form of figures and tables. In line with the concept of waste or by-product, there are more reports of nacre extract than pearl extract, due to the expensive and limited in origin of pearls. Mainly on the direct use of nacre powder and pearl powder or on the use of extracts (mainly water soluble proteins) through experiments in vivo or in vitro, and shows whether it is effective through the results of various indexes. There is no further study on substances other than extracts, and the structural analysis of extracts needs further exploration.

A shell matrix protein of Pinctada mazatlanica produces nacre platelets in vitro

Nacre is the main component of the pearl oyster shells and it is synthesized by specialized soluble and insoluble shell matrix proteins. Insoluble proteins from the decalcification of the shell are the less studied proteins due to the technical problems to isolate them from the organic matrix. In this study, an insoluble shell matrix protein from Pinctada mazatlanica, pearlin (Pmaz-pearlin), was successfully cloned from the mantle tissue, and the native protein isolated from the shell was functionally characterized. The full coding sequence of Pmaz-pearlin mRNA consists of 423 base pairs, which encode to a 16.3 kDa pearlin. Analysis of the deduced amino acid sequence revealed that Pmaz-pearlin contained four acidic regions, an NG repeat domain, and Cys conserved residues, the latter potentially forms four disulfide bridges which might stabilize the protein structure. The isolated protein from the shell is a glycoprotein of ~ 16.74 kDa which can produce aragonite and calcite crystals in vitro. Our results show that Pmaz-pearlin is a well-conserved protein involved in nacre layer growth, which produces calcite crystals in the presence of CaCl2, aragonite crystal polymorphs with a hexagonal structure in the presence of MgCl2, and needle-like crystal structure polymorphs in the presence of CaCO3 The identity of the crystals was confirmed using RAMAN analyses.

Key Insights, Tools, and Future Prospects on Oyster Shell End-of-Life: A Critical Analysis of Sustainable Solutions

Oyster farming represents one of the most developed aquaculture activities, producing delicacies unfortunately related to a direct accumulation of waste shells. Facing what is becoming an environmental issue, chemists are currently developing solutions to add value to this wild source of raw material in line with the principles of sustainable chemistry. An argumentative overview of this question is proposed here with a focus on recent data. Starting with a presentation of the environmental impact of oyster farming, existing and promising applications are then classified according to the type of raw materials derived from the oyster shell, namely the natural oyster shell (NOS), the calcined natural oyster shell (CNOS), and biomolecules of the organic matrix extracted from the oyster shell. Their relevance is discussed in regard to their scalability, originality, and sustainability. This review constitutes the first critical compilation on oyster shell applications, with the aim to provide essential elements to better comprehend the recycling of waste oyster shells.

Electroextraction of Insoluble Proteins from the Organic Matrix of the Nacreous Layer of the Japanese Pearl Oyster, Pinctada fucata

The nacreous layer of shells and pearls is composed of aragonite crystals arranged in an organic matrix. The organic matrix contains chitin and several proteins that regulate the formation of the nacreous layer. Owing to their strong interactions in the organic matrix, the current method for extraction of insoluble proteins from the pre-powdered nacreous layer involves heating to high temperatures in the presence of a detergent (e.g., sodium dodecyl sulfate, SDS) and reductant (e.g., dithiothreitol, DTT), which is likely to induce protein degradation. Therefore, we have developed an electroextraction method to isolate proteins from the organic matrix of a nacreous organic sheet, that was obtained following the decalcification of shells in their original shape. Our electroextraction method employs milder conditions without heating or detergent. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) patterns of the electro-extracted proteins (EEPs) under non-reduced and reduced conditions revealed that this method yielded a greater number of different proteins compared with the conventional extraction method and the isolated EEPs retained their disulfide bonds. Our method is able to easily extract insoluble proteins from the nacreous layer under mild conditions and will undoubtedly aid future analyses into the functions of the nacreous layer proteins.

Variation in Orthologous Shell-Forming Proteins Contribute to Molluscan Shell Diversity

Despite the evolutionary success and ancient heritage of the molluscan shell, little is known about the molecular details of its formation, evolutionary origins, or the interactions between the material properties of the shell and its organic constituents. In contrast to this dearth of information, a growing collection of molluscan shell-forming proteomes and transcriptomes suggest they are comprised of both deeply conserved, and lineage specific elements. Analyses of these sequence data sets have suggested that mechanisms such as exon shuffling, gene co-option, and gene family expansion facilitated the rapid evolution of shell-forming proteomes and supported the diversification of this phylum specific structure. In order to further investigate and test these ideas we have examined the molecular features and spatial expression patterns of two shell-forming genes (Lustrin and ML1A2) and coupled these observations with materials properties measurements of shells from a group of closely related gastropods (abalone). We find that the prominent “GS” domain of Lustrin, a domain believed to confer elastomeric properties to the shell, varies significantly in length between the species we investigated. Furthermore, the spatial expression patterns of Lustrin and ML1A2 also vary significantly between species, suggesting that both protein architecture, and the regulation of spatial gene expression patterns, are important drivers of molluscan shell evolution. Variation in these molecular features might relate to certain materials properties of the shells of these species. These insights reveal an important and underappreciated source of variation within shell-forming proteomes that must contribute to the diversity of molluscan shell phenotypes.

Shell extracts of the edible mussel and oyster induce an enhancement of the catabolic pathway of human skin fibroblasts, in vitro

Mollusc shells are composed of more than 95% calcium carbonate and less than 5% organic matrix consisting mostly of proteins, glycoproteins and polysaccharides. In this study, we investigated the effects of matrix macromolecular components extracted from the shells of two edible molluscs of economic interest, i.e., the blue mussel Mytilus edulis and the Pacific oyster Crassostrea gigas. The potential biological activities of these organic molecules were analysed on human dermal fibroblasts in primary culture. Our results demonstrate that shell extracts of the two studied molluscs modulate the metabolic activities of the cells. In addition, the extracts caused a decrease of type I collagen and a concomitant increase of active MMP-1, both at the mRNA and the protein levels. Therefore, our results suggest that shell extracts from M. edulis and C. gigas contain molecules that promote the catabolic pathway of human dermal fibroblasts. This work emphasises the potential use of these shell matrices in the context of anti-fibrotic strategies, particularly against scleroderma. More generally, it stresses the usefulness to valorise bivalve shells that are coproducts of shellfish farming activity.

Principles of calcium-based biomineralization

The chapter provides some basic information on the formation principles of calcium carbonate in biological systems in marine environment in the point of view of materials science in order to provide strategies for biomimetic design and preparation of new functional materials. Many researchers try to explain the principles of biomineralization and get some valuable conclusions. This chapter introduces some calcium-based biominerals in aquatic organisms which mainly include calcium carbonate and calcium phosphate. Then it gives a presentation of the hierarchical structure of calcium carbonate-based and calcium phosphate-based biominerals, e.g., mollusc shell, pearl, carp otolith, tooth, and bone. Moreover, the chapter explains the principles of calcium carbonate mineralization from the aspects of the effects of additives and templates; it also gives some explanations to the principles of calcium phosphate mineralization.

Biomineralization of Schlumbergerella floresiana, a significant carbonate-producing benthic foraminifer

Most foraminifera that produce a shell are efficient biomineralizers. We analyzed the calcitic shell of the large tropical benthic foraminifer Schlumbergerella floresiana. We found a suite of macromolecules containing many charged and polar amino acids and glycine that are also abundant in biomineralization proteins of other phyla. As neither genomic nor transcriptomic data are available for foraminiferal biomineralization yet, de novo-generated sequences, obtained from organic matrices submitted to ms blast database search, led to the characterization of 156 peptides. Very few homologous proteins were matched in the proteomic database, implying that the peptides are derived from unknown proteins present in the foraminiferal organic matrices. The amino acid distribution of these peptides was queried against the uniprot database and the mollusk uniprot database for comparison. The mollusks compose a well-studied phylum that yield a large variety of biomineralization proteins. These results showed that proteins extracted from S. floresiana shells contained sequences enriched with glycine, alanine, and proline, making a set of residues that provided a signature unique to foraminifera. Three of the de novo peptides exhibited sequence similarities to peptides found in proteins such as pre-collagen-P and a group of P-type ATPases including a calcium-transporting ATPase. Surprisingly, the peptide that was most similar to the collagen-like protein was a glycine-rich peptide reported from the test and spine proteome of sea urchin. The molecules, identified by matrix-assisted laser desorption ionization-time of flight mass spectrometry analyses, included acid-soluble N-glycoproteins with its sugar moieties represented by high-mannose-type glycans and carbohydrates. Describing the nature of the proteins, and associated molecules in the skeletal structure of living foraminifera, can elucidate the biomineralization mechanisms of these major carbonate producers in marine ecosystems. As fossil foraminifera provide important paleoenvironmental and paleoclimatic information, a better understanding of biomineralization in these organisms will have far-reaching impacts.

Shell extracts from the marine bivalve Pecten maximus regulate the synthesis of extracellular matrix in primary cultured human skin fibroblasts

Mollusc shells are composed of more than 95% calcium carbonate and less than 5% of an organic matrix consisting mostly of proteins, glycoproteins and polysaccharides. Previous studies have elucidated the biological activities of the shell matrices from bivalve molluscs on skin, especially on the expression of the extracellular matrix components of fibroblasts. In this work, we have investigated the potential biological activities of shell matrix components extracted from the shell of the scallop Pecten maximus on human fibroblasts in primary culture. Firstly, we demonstrated that shell matrix components had different effects on general cellular activities. Secondly, we have shown that the shell matrix components stimulate the synthesis of type I and III collagens, as well as that of sulphated GAGs. The increased expression of type I collagen is likely mediated by the recruitment of transactivating factors (Sp1, Sp3 and human c-Krox) in the −112/−61 bp COL1A1 promoter region. Finally, contrarily to what was obtained in previous works, we demonstrated that the scallop shell extracts have only a small effect on cell migration during in vitro wound tests and have no effect on cell proliferation. Thus, our research emphasizes the potential use of shell matrix of Pecten maximus for dermo-cosmetic applications.

Shotgun proteomics analysis of proliferating STRO-1-positive human dental pulp cell after exposure to nacreous water-soluble matrix

INTRODUCTION

For dental treatment, dentin regeneration is required after a tooth injury with dental pulp exposure. The effects of the water-soluble matrix (WSM) extracted from the nacreous layer of the bivalve Pinctada maxima on human dental pulp cells in vitro were challenging and useful for clinical application.

MATERIAL AND METHODS

The biological activity of the STRO-1-positive human dental pulp cells in response to WSM compared to Dulbecco's modified Eagle medium (DMEM) as a normal control was monitored. The cell survival rate was analyzed by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Proteomic profiles among inducers and noninducers with time dependency were compared by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis combined with liquid chromatography-tandem mass spectrometry (GeLC-MS/MS).

RESULTS

The human dental pulp cells cultured in nacreous WSM exhibited higher relative cell viability than those in DMEM with similar morphological appearance. Significant changes were found in the relative abundance of 44 proteins in cells after exposure to WSM for 2 weeks. They play a role in cell adhesion, cell proliferation, metabolic process, signal transduction, stress response, transcription, translation, and transport.

CONCLUSION

These results indicate that WSM of P. maxima has the ability to induce proliferation of human dental pulp cells.

CLINICAL RELEVANCE

This finding initiated the study to evaluate the suitability of nacre as biomaterial for dentistry.

Water soluble bioactives of nacre mediate antioxidant activity and osteoblast differentiation

The water soluble matrix of nacre is a proven osteoinductive material. In spite of the differences in the biomolecular compositions of nacre obtained from multiple species of oysters, the common biochemical properties of those principles substantiate their biological activity. However, the mechanism by which nacre stimulates bone differentiation remains largely unknown. Since the positive impact of antioxidants on bone metabolism is well acknowledged, in this study we investigated the antioxidant potential of a water soluble matrix (WSM) obtained from the nacre of the marine oyster Pinctada fucata, which could regulate its osteoblast differentiation activity. Enhanced levels of ALP activity observed in pre-osteoblast cells upon treatment with WSM, suggested the induction of bone differentiation events. Furthermore, bone nodule formation and up-regulation of bone differentiation marker transcripts, i.e. collagen type-1 and osteocalcin by WSM confirmed its ability to induce differentiation of the pre-osteoblasts into mature osteoblasts. Remarkably, same WSM fraction upon pre-treatment lowered the H2O2 and UV-B induced oxidative damages in keratinocytes, thus indicating the antioxidant potential of WSM. This was further confirmed from the in vitro scavenging of ABTS and DPPH free radicals and inhibition of lipid peroxidation by WSM. Together, these results indicate that WSM poses both antioxidant potential and osteoblast differentiation property. Thus, bioactivities associated with nacre holds potential in the development of therapeutics for bone regeneration and against oxidative stress induced damages in cells.

Identification of two carbonic anhydrases in the mantle of the European Abalone Haliotis tuberculata (Gastropoda, Haliotidae): phylogenetic implications

Carbonic anhydrases (CAs) represent a diversified family of metalloenzymes that reversibly catalyze the hydration of carbon dioxide. They are involved in a wide range of functions, among which is the formation of CaCO(3) skeletons in metazoans. In the shell-forming mantle tissues of mollusks, the location of the CA catalytic activity is elusive and gives birth to contradicting views. In the present paper, using the European abalone Haliotis tuberculata, a key model gastropod in biomineralization studies, we identified and characterized two CAs (htCA1 and htCA2) that are specific of the shell-forming mantle tissue. We analyzed them in a phylogenetic context. Combining various approaches, including proteomics, activity tests, and in silico analyses, we showed that htCA1 is secreted but is not incorporated in the organic matrix of the abalone shell and that htCA2 is transmembrane. Together with previous studies dealing with molluskan CAs, our findings suggest two possible modes of action for shell mineralization: the first mode applies to, for example, the bivalves Unio pictorum and Pinctada fucata, and involves a true CA activity in their shell matrix; the second mode corresponds to, for example, the European abalone, and does not include CA activity in the shell matrix. Our work provides new insight on the diversity of the extracellular macromolecular tools used for shell biomineralization study in mollusks.

Biosilicification of loricate choanoflagellate: organic composition of the nanotubular siliceous costal strips of Stephanoeca diplocostata

Loricate choanoflagellates (unicellular, eukaryotic flagellates; phylum Choanozoa) synthesize a basket-like siliceous lorica reinforced by costal strips (diameter of approximately 100 nm and length of 3 μm). In the present study, the composition of these siliceous costal strips is described, using Stephanoeca diplocostata as a model. Analyses by energy-dispersive X-ray spectroscopy (EDX), coupled with transmission electron microscopy (TEM), indicate that the costal strips comprise inorganic and organic components. The organic, proteinaceous scaffold contained one major polypeptide of mass 14 kDa that reacted with wheat germ agglutinin. Polyclonal antibodies were raised that allowed mapping of the proteinaceous scaffold, the (glyco)proteins, within the costal strips. Subsequent in vitro studies revealed that the organic scaffold of the costal strips stimulates polycondensation of ortho-silicic acid in a concentration- and pH-dependent way. Taken together, the data gathered indicate that the siliceous costal strips are formed around a proteinaceous scaffold that supports and maintains biosilicification. A scheme is given that outlines that the organic template guides both the axial and the lateral growth of the strips.

Effects of pearl powder extract and its fractions on fibroblast function relevant to wound repair

The water soluble matrix (WSM) of pearl powder [Hyriopsis cumingii Lea (Unionidae)] was extracted, and the insoluble residue was demineralized, size-fractionated, and named as MR14 (> 14 kDa), MR3-14 (3-14 kDa), and MR3 (< 3 kDa). The effects of WSM, MR14, MR3-14, and MR3 on primary mouse oral fibroblast proliferation, collagen accumulation, matrix metalloproteinase-2, -9 (MMP-2, -9) activities, and tissue inhibitor of metalloproteinase-1 (TIMP-1) production were tested by MTT assay, chloramine T method, gelatin zymography, and enzyme-linked immunosorbent assay (ELISA), respectively. The results showed that the WSM and MR14 could significantly (p < 0.05) promote fibroblast proliferation; all of the fractions could significantly promote collagen accumulation; MR14 significantly (p < 0.05) inhibited MMP-2 activity; and all of the fractions could significantly promote TIMP-1 production. This study has proved that the mechanism by which pearl powder promotes wound healing is partly due to its ability to stimulate fibroblast mitosis, collagen deposition, and TIMP-1 production, and the major active fraction may be MR14.

Evolution of nacre: biochemistry and proteomics of the shell organic matrix of the cephalopod Nautilus macromphalus

In mollusks, one of the most widely studied shell textures is nacre, the lustrous aragonitic layer that constitutes the internal components of the shells of several bivalves, a few gastropods,and one cephalopod: the nautilus. Nacre contains a minor organic fraction, which displays a wide range of functions in relation to the biomineralization process. Here, we have biochemically characterized the nacre matrix of the cephalopod Nautilus macromphalus. The acid-soluble matrix contains a mixture of polydisperse and discrete proteins and glycoproteins, which interact with the formation of calcite crystals. In addition, a few bind calcium ions. Furthermore, we have used a proteomic approach,which was applied to the acetic acid-soluble and -insoluble shell matrices, as well as to spots obtained after 2D gel electrophoresis. Our data demonstrate that the insoluble and soluble matrices, although different in their bulk monosaccharide and amino acid compositions, contain numerous shared peptides. Strikingly, most of the obtained partial sequences are entirely new. A few only partly match with bivalvian nacre proteins.Our findings have implications for knowledge of the long-term evolution of molluskan nacre matrices.

Quantitative expression analysis of nacreous shell matrix protein genes in the process of pearl biogenesis

A cultured pearl is produced in a pearl sac which forms layers of cells differentiated from an allograft mantle. Previous investigations claimed that genomic DNAs from grafting tissues were persistent during pearl aquaculture. However, the specific living status of the genes regulating pearl formation has remained unknown. This study examined the expression profiles of six genes encoding nacreous shell matrix proteins (NSMPs) in the pearl sac of aquaculture pearl oyster Pinctada fucata by real-time PCR. The comparative analysis of NSMP gene expression in the pearl sac and reference mantle tissues revealed that only a few NSMP genes maintained high transcription levels in the pearl sac. The impaired transcription levels of the nacrein gene refreshed the previous hypothesis, suggesting that CaCO(3) required for pearl secretion was not from pearl sac cells. Among the examined genes, only the N19 gene attained high expression levels in the pearl sac. These results provide new insights into the molecular mechanisms involved in pearl biogenesis.

A novel nacre protein N19 in the pearl oyster Pinctada fucata

A novel 19kDa protein, which was named N19, was isolated from the nacreous layer of the pearl oyster Pinctada fucata. N19 is one of predominant proteins found in the water-insoluble fraction of the nacreous layer. MALDI-TOF/TOF analysis indicated that the three trypsin-digested peptides (791.45, 824.42, and 1118.65m/z) corresponded to the amino acid sequences predicted from a cDNA isolated from a mantle cDNA library of P. fucata. Northern blot analysis revealed that the N19 mRNA was a little more abundant in the pallial region than the edge region, in the mantle. In CaCO(3) precipitation assay, the recombinant N19 protein inhibited the crystallization of CaCO(3). These results indicate that N19 is localized in the nacre and plays a negative regulatory role in calcification in the pearl oyster.

Characterization and in vitro mineralization function of a soluble protein complex P60 from the nacre of Pinctada fucata

A soluble protein complex P60 from the powdered nacre of Pinctada fucata was extracted and partially characterized. The biological activity of the P60 on pre-osteoblast cell line MC3T3-E1 and bone marrow stroma cells (MSCs) was investigated. The P60 protein from the decalcified powered nacre was solubilized with acetic acid and then purified by liquid chromatography. The P60 protein was a protein complex composed of several subunits with disulfide bridges. The known protein nacrein, and its two derivatives, N28 and N35, were included in the P60 protein complex. The most abundant amino acids in the P60 that account for 68.3% of the total residues are glycine (32.1%), aspartic acid (17.4%), alanine (13.6%), and glutamic acid (5.2%). The in vitro study of the crystallization showed that this protein complex could control the formation and size of calcium carbonate. The assay of biological activity of the P60 protein complex on the pre-osteoblast cell line MC3T3-E1 and MSCs suggested that the P60 could stimulate the formation of mineralized nodules.

The shell matrix of the freshwater mussel Unio pictorum (Paleoheterodonta, Unionoida). Involvement of acidic polysaccharides from glycoproteins in nacre mineralization

Among molluscs, the shell biomineralization process is controlled by a set of extracellular macromolecular components secreted by the calcifying mantle. In spite of several studies, these components are mainly known in bivalves from only few members of pteriomorph groups. In the present case, we investigated the biochemical properties of the aragonitic shell of the freshwater bivalve Unio pictorum (Paleoheterodonta, Unionoida). Analysis of the amino acid composition reveals a high amount of glycine, aspartate and alanine in the acid-soluble extract, whereas the acid-insoluble one is rich in alanine and glycine. Monosaccharidic analysis indicates that the insoluble matrix comprises a high amount of glucosamine. Furthermore, a high ratio of the carbohydrates of the soluble matrix is sulfated. Electrophoretic analysis of the acid-soluble matrix revealed discrete bands. Stains-All, Alcian Blue, periodic acid/Schiff and autoradiography with (45)Ca after electrophoretic separation revealed three major polyanionic calcium-binding glycoproteins, which exhibit an apparent molecular mass of 95, 50 and 29 kDa, respectively. Two-dimensional gel electrophoresis shows that these bands, provisionally named P95, P50 and P29, are composed of numerous isoforms, the majority of which have acidic isoelectric points. Chemical deglycosylation of the matrix with trifluoromethanesulfonic acid induces a drastic shift of both the apparent molecular mass and the isoelectric point of these matrix components. This treatment induces also a modification of the shape of CaCO(3) crystals grown in vitro and a loss of the calcium-binding ability of two of the main matrix proteins (P95 and P50). Our findings strongly suggest that post-translational modifications display important functions in mollusc shell calcification.

Molluscan shell proteins: primary structure, origin, and evolution

In the last few years, the field of molluscan biomineralization has known a tremendous mutation, regarding fundamental concepts on biomineralization regulation as well as regarding the methods of investigation. The most recent advances deal more particularly with the structure of shell biominerals at nanoscale and the identification of an increasing number of shell matrix protein components. Although the matrix is quantitatively a minor constituent in the shell of mollusks (less than 5% w/w), it is, however, the major component that controls different aspects of the shell formation processes: synthesis of transient amorphous minerals and evolution to crystalline phases, choice of the calcium carbonate polymorph (calcite vs aragonite), organization of crystallites in complex shell textures (microstructures). Until recently, the classical paradigm in molluscan shell biomineralization was to consider that the control of shell synthesis was performed primarily by two antagonistic mechanisms: crystal nucleation and growth inhibition. New concepts and emerging models try now to translate a more complex reality, which is remarkably illustrated by the wide variety of shell proteins, characterized since the mid-1990s, and described in this chapter. These proteins cover a broad spectrum of pI, from very acidic to very basic. The primary structure of a number of them is composed of different modules, suggesting that these proteins are multifunctional. Some of them exhibit enzymatic activities. Others may be involved in cell signaling. The oldness of shell proteins is discussed, in relation with the Cambrian appearance of the mollusks as a mineralizing phylum and with the Phanerozoic evolution of this group. Nowadays, the extracellular calcifying shell matrix appears as a whole integrated system, which regulates protein-mineral and protein-protein interactions as well as feedback interactions between the biominerals and the calcifying epithelium that synthesized them. Consequently, the molluscan shell matrix may be a source of bioactive molecules that would offer interesting perspectives in biomaterials and biomedical fields.

Matrix proteins in the outer shells of molluscs

The shells of molluscs are composed mainly of calcium carbonate crystals, with small amounts of matrix proteins. For more than 50 years, they have attracted attention for their unique mechanical and biological properties. Only recently, however, have researchers begun to realize that it is the matrix proteins that control the formation of calcium carbonate crystals and play key roles in their extraordinary properties, despite the fact that matrix proteins comprise less than 5% of the shell weight. This article reviews the matrix proteins identified to date from the shells of molluscs, their structural characteristics, and their roles in shell formation. Some suggestions are given for further investigation based on the summary and analysis.

Shematrin: A family of glycine-rich structural proteins in the shell of the pearl oyster Pinctada fucata

Random sequencing of molecules from a cDNA library constructed from mantle mRNA of the pearl oyster Pinctada fucata was used to obtain information on organic matrix proteins in the shell. In the determined sequences, we identified 7 distinct cDNAs encoding similar glycine-rich domains. Complete sequence analysis of these cDNAs showed that the predicted sequences of the proteins, which we named shematrins, possessed similar domains comprising repeat sequences of two or more glycines, followed by a hydrophobic amino acid. In addition, in shematrin-1, -2 and -3, a repeat domain designated as XGnX (where X is a hydrophobic amino acid) was conserved. It is of further note that all the shematrin proteins have RKKKY, RRKKY or RRRKY as their C-terminal sequence. According to northern blot analysis, all shematrins are exclusively expressed in the mantle, and particularly in the edge region of the mantle; furthermore, peptide fragments similar to shematrin-1 and -2 were detected in the prismatic layer of shells by MALDI-TOF/TOF MS analysis. These findings suggest that many of shematrins are synthesized in the mantle edge and secreted into the prismatic layer of the shell, where the protein family is thought to provide a framework for calcification.

Caspartin and Calprismin, Two Proteins of the Shell Calcitic Prisms of the Mediterranean Fan Mussel Pinna nobilis

We used the combination of preparative electrophoresis and immunological detection to isolate two new proteins from the shell calcitic prisms of Pinna nobilis, the Mediterranean fan mussel. The amino acid composition of these proteins was determined. Both proteins are soluble, intracrystalline, and acidic. The 38-kDa protein is glycosylated; the 17-kDa one is not. Ala, Asx, Thr, and Pro represent the dominant residues of the 38-kDa protein, named calprismin. An N-terminal sequence was obtained from calprismin. This sequence, which comprises a pattern of 4 cysteine residues, is not related to any known protein. The second protein, named caspartin, exhibits an unusual amino acid composition, since Asx constitutes by far the main amino acid residue. Preliminary sequencing surprisingly suggests that the first 75 N-terminal residues are all Asp. Caspartin self-aggregates spontaneously into multimers. In vitro tests show that it inhibits the precipitation of calcium carbonate. Furthermore, it strongly interferes with the growth of calcite crystals. A polyclonal antiserum raised against caspartin was used to localize this protein in the shell by immunogold. The immunolocalization demonstrates that caspartin is distributed within the prisms and makes a continuous film at the interface between the prisms and the surrounding insoluble sheets. Our finding emphasizes the prominent role of aspartic acid-rich proteins for the building of calcitic prisms among molluscs.

Soluble organic matrix of two Scleractinian corals: Partial and comparative analysis

This study is a biochemical and molecular analysis of the soluble organic matrix (SOM) of two Scleractinian corals differing in their morphological characteristics: Stylophora pistillata, a branched robust coral and Pavona cactus, a leafy complex coral. Soluble organic matrix of both coral species were shown to contain high amounts of potentially acidic amino acids and glycine. However, proportions of glycosaminoglycans and SDS-PAGE analyses of soluble organic matrix proteins were very different. Three proteins of S. pistillata and at least five proteins of P. cactus were detected by silver staining, some of them being able to bind calcium. Internal peptide sequences of two matrix proteins (one from each species) were obtained. One sequence of S. pistillata is unusual because it contains a long poly-aspartate domain, as described in proteins belonging to the calsequestrin family and in proteins from molluscan species. This domain suggests an essential role for this protein in the control of mineralization.

Structure−Function Relationship of Avian Eggshell Matrix Proteins: A Comparative Study of Two Major Eggshell Matrix Proteins, Ansocalcin and OC-17

The role of individual matrix proteins in avian eggshell calcification is poorly understood despite numerous attempts to characterize and localize their presence in the eggshell matrix. Ansocalcin, the major matrix protein from goose eggshell, was found to induce the formation of calcite crystal aggregates under in vitro. Owing to its high similarity with the chicken eggshell matrix protein ovocleidin 17 (OC-17), a comparative investigation has been carried out to understand the structure-function relationship. RP-HPLC shows that ansocalcin is the major component in extracts of goose eggshells before and after bleach treatment. However, OC-17 was observed in minute quantities in the extract of bleach-treated chicken eggshells. In vitro crystal growth experiments showed that OC-17 and ansocalcin interact differently with the calcite crystals formed. Circular dichroism, intrinsic tryptophan fluorescence, and dynamic light scattering studies showed that, under the conditions used in our experiments, OC-17 does not aggregate in solution or induce the nucleation of calcite aggregates in the concentration range used. These observations indicate that OC-17 and ansocalcin play different roles in the eggshell calcification. To our knowledge, this is the first report on the comparison of properties of homologous eggshell proteins that belong to the same phylogeny.

Zona localization of shell matrix proteins in mantle of Haliotis tuberculata (Mollusca, Gastropoda)

Organic matrix from molluscan shells has the potential to regulate calcium carbonate deposition and crystallization. Control of crystal growth thus seems to depend on control of matrix protein secretion or activation processes in the mantle cells, about which little is known. Biomineralization is a highly orchestrated biological process. The aim of this work was to provide information about the source of shell matrix macromolecule production, within the external epithelium of the mantle. An in vivo approach was chosen to describe the histologic changes in the outer epithelium and in blood sinus distribution, associated with mantle cells implicated in shell matrix production. Our results characterized a topographic and time-dependent zonation of matrix proteins involved in shell biomineralization in the mantle of Haliotis.

Acharan sulfate, the new glycosaminoglycan from Achatina fulica Bowdich 1822. Structural heterogeneity, metabolic labeling and localization in the body, mucus and the organic shell matrix

Acharan sulfate, a recently discovered glycosaminoglycan isolated from Achatina fulica, has a major disaccharide repeating unit of -->4)-2-acetyl,2-deoxy-alpha-d-glucopyranose(1-->4)-2-sulfo-alpha-l-idopyranosyluronic acid (1-->, making it structurally related to both heparin and heparan sulfate. It has been suggested that this glycosaminoglycan is polydisperse, with an average molecular mass of 29 kDa and known minor disaccharide sequence variants containing unsulfated iduronic acid. Acharan sulfate was found to be located in the body of this species using alcian blue staining and it was suggested to be the main constituent of the mucus. In the present work, we provide further information on the structure and compartmental distribution of acharan sulfate in the snail body. Different populations of acharan sulfate presenting charge and/or molecular mass heterogeneities were isolated from the whole body, as well as from mucus and from the organic shell matrix. A minor glycosaminoglycan fraction susceptible to degradation by nitrous acid was also purified from the snail body, suggesting the presence of N-sulfated glycosaminoglycan molecules. In addition, we demonstrate the in vivo metabolic labeling of acharan sulfate in the snail body after a meal supplemented with [35S]free sulfate. This simple approach might be applied to the study of acharan sulfate biosynthesis. Finally, we developed histochemical assays to localize acharan sulfate in the snail body by metachromatic staining and by histoautoradiography following metabolic radiolabeling with [35S]sulfate. Our results show that acharan sulfate is widely distributed among several organs.

Characterization of two molluscan crystal-modulating biomineralization proteins and identification of putative mineral binding domains

Ethylenediamine-tetraacetic acid extracted water-soluble matrix proteins in molluscan shells secreted from the mantle epithelia are believed to control crystal nucleation, morphology, orientation, and phase of the deposited mineral. Previously, atomic force microscopy demonstrated that abalone nacre proteins bind to growing step edges and to specific crystallographic faces of calcite, suggesting that inhibition of calcite growth may be one of the molecular processes required for growth of the less thermodynamically stable aragonite phase. Previous experiments were done with protein mixtures. To elucidate the role of single proteins, we have characterized two proteins isolated from the aragonitic component of nacre of the red abalone, Haliotis rufescens. These proteins, purified by hydrophobic interaction chromatography, are designated AP7 and AP24 (aragonitic protein of molecular weight 7 kDa and 24 kDa, respectively). Degenerate oligonucleotide primers corresponding to N-terminal and internal peptide sequences were used to amplify cDNA clones by a polymerase chain reaction from a mantle cDNA library; the deduced primary amino acid sequences are presented. Preliminary crystal growth experiments demonstrate that protein fractions enriched in AP7 and AP24 produced CaCO(3) crystals with morphology distinct from crystals grown in the presence of the total mixture of soluble aragonite-specific proteins. Peptides corresponding to the first 30 residues of the N-terminal sequences of both AP7 and AP24 were generated. The synthetic peptides frustrate the progression of step edges of a growing calcite surface, indicating that sequence features within the N-termini of AP7 and AP24 include domains that interact with CaCO(3). CD analyses demonstrate that the N-terminal peptide sequences do not possess significant percentages of alpha-helix or beta-strand secondary structure in solution. Instead, in both the presence and absence of Ca(II), the peptides retain unfolded conformations that may facilitate protein-mineral interaction.

Development of calcareous skeletal elements in invertebrates

Most metazoans require skeletal support systems. While the formation of bones and teeth in vertebrates has been well studied, endo- and exoskeleton development of non-vertebrates, especially calcification during terminal differentiation, has been neglected. Biomineralization of skeletons in invertebrates presents interesting research opportunities. We undertake here to survey some of the better understood examples of skeletal development in selected invertebrates. The differentiation of the skeletal spicules of euechinoid larvae and other non-vertebrate deuterostomes, the shells of molluscs, and the calcification of crustacean carapaces are surveyed. The diversity of these different kinds of animals and our present limited understanding make it difficult to identify unifying themes, but there certainly are unifying questions: How is the mineral precursor secreted? What is the nature of the interaction of mineral with the matrix proteins of the skeleton? Is there any conservation of protein domains in matrix proteins found in skeletal elements from different phyla? Are there common strategies in the development of organs that form mineralized structures?

Bioactivity of nacre water-soluble organic matrix from the bivalve mollusk Pinctada maxima in three mammalian cell types: fibroblasts, bone marrow stromal cells and osteoblasts

In vivo and in vitro studies provide strong evidence of the osteogenic activity of nacre obtained from Pinctada maxima. The in vitro studies indicate that diffusible factors from nacre are involved in cell stimulation. The water-soluble matrix (WSM) was extracted from nacre by a non-decalcifying process, and four fractions (SE(1)-SE(4)) were separated by SE-HPLC. Those fractions were tested in vitro on MRC5 fibroblasts. Alkaline phosphatase (ALP) activity was measured as a marker of osteoblastic differentiation. The anti-apoptotic protein Bcl-2 was also immunodetected in cultured osteoblasts from rat calvaria. WSM and fraction SE(4) increased ALP activity. BMP-2 had the same effect on the cells as WSM and SE(4). WSM greatly increased the amount of Bcl-2 in the cytoplasm and nucleus of osteoblasts. These in vitro studies support our initial hypothesis that nacre organic matrix (WSM) of a bivalve mollusk contains signal-molecules that can stimulate the osteogenic pathway in mammalian cells that are targets for bone induction.