Lipids from the nacreous and prismatic layers of two Pteriomorpha mollusc shells

Similar construction of spicules and shell plates: Implications for the origin of chiton biomineralization

The hard shells of mollusks are products of biomineralization, a distinctive feature of the Cambrian explosion. Despite our understanding of shell structure and mechanical properties, their origin remains mysterious. In addition to their shell plates, most chitons have calcium deposits on their girdles. However, the similarity of these two mineralized structures still needs to be determined, limiting our comprehension of their origins. In our study, we analyzed the matrix proteins in the spicules of chiton (Acanthopleura loochooana) and compared them with the matrix proteins in the shells of the same species. Proteomics identified 96 unique matrix proteins in spicules. Comparison of biomineralization-related matrix proteins in shell plates and spicules revealed shared proteins, including carbonic anhydrases, tyrosinase-hemocyanin, von Willebrand factor type A, cadherin, and glycine-rich unknown proteins. Based on similarities in key matrix proteins, we propose that spicules and shell plates originated from a common mineralization system in their ancestral lineage, suggesting the existence of a common core or toolkit of matrix proteins among calcifying organisms. SIGNIFICANCE: In this study, we try to understand the types and diversity of matrix proteins in the biomineralization of chiton shell plates and spicules. Through a comparative analysis, we seek insights into the core biomineralization toolkit of ancestral mollusks. To achieve this, we conducted LC-MS/MS and RT-qPCR analyses to identify the types and relative expression levels of matrix proteins in both shell plates and spicules. The analysis revealed 96 matrix proteins in the spicules. A comparison of biomineralization-related matrix proteins in shell plates and spicules from the same species revealed shared proteins including many unknown proteins unique to chitons. Blast searching reveals a universal conservation of these proteins among other chitons. Hence, we propose that spicules and shell plates originated from a common mineralization system in their ancestral lineage. Our work provides a molecular basis for studying biomineralization in polyplacophoran mollusks and understanding biomineralization evolution. In addition, it identifies potential matrix proteins that could be applied to control crystal growth.

Stable isotopes in the shell organic matrix for (paleo)environmental reconstructions

Stable isotope ratios of mollusc shell carbonates have long been used to reconstruct past environmental conditions. Although shells also contain organics, they are seldom used in (paleo)climatic studies. Here, we extract the acid-soluble and insoluble fractions of the organic matrix of modern Mytilus galloprovincialis shells from three sites along a coast-to-upper-estuary environmental gradient to measure their hydrogen (δ2H) and oxygen (δ18O) isotope compositions. Both organic fractions showed isotopic signatures significantly different from those of carbonate and water at each site, indicating the involvement of different fractionation mechanisms. The soluble fraction showed gradual differences in isotope values along the gradient, while the insoluble fraction showed δ2H-δ18O correlation regressions subparallel to the Global and Local Meteoric Water Lines. These results showed the great potential of the shell organic matrix stable isotopes as possible (paleo)environmental proxies, stimulating further research to better define the fractionation mechanisms involved.

The biocide triclosan as a potential developmental disruptor in Mytilus early larvae

The broadly utilized biocide triclosan (TCS) is continuously discharged in water compartments worldwide, where it is detected at concentrations of ng-µg/L. Given its lipophilicity and bioaccumulation, TCS is considered potentially harmful to human and environmental health and also as a potential endocrine disruptor (ED) in different species. In aquatic organisms, TCS can induce a variety of effects: however, little information is available on its possible impact on invertebrate development. Early larval stages of the marine bivalve Mytilus galloprovincialis have been shown to be sensitive to environmental concentrations of a number of emerging contaminants, including EDs. In this work, the effects of TCS were first evaluated in the 48 h larval assay in a wide concentration range (0.001-1,000 μg/L). TCS significantly affected normal development of D-veligers (LOEC = 0.1 μg/L; EC50 = 236.1 μg/L). At selected concentrations, the mechanism of action of TCS was investigated. TCS modulated transcription of different genes involved in shell mineralization, endocrine signaling, ceramide metabolism, and biotransformation, depending on larval stage (24 and 48 h post-fertilization-hpf) and concentration (1 and 10 μg/L). At 48 hpf and 10 μg/L TCS, calcein staining revealed alterations in CaCO3 deposition, and polarized light microscopy showed the absence of shell birefringence due to the mineralized phase. Observations by scanning electron microscopy highlighted a variety of defects in shell formation from concentrations as low as 0.1 μg/L. The results indicate that TCS, at environmental exposure levels, can act as a developmental disruptor in early mussel larvae mainly by interfering with the processes of biomineralization.

Effects of ocean acidification and tralopyril on bivalve biomineralization and carbon cycling: A study of the Pacific Oyster (Crassostrea gigas)

The combined effects of emerging pollutants and ocean acidification (OA) on marine organisms and marine ecosystems have attracted increasing attention. However, the combined effects of tralopyril and OA on marine organisms and marine ecosystems remain unclear. In this study, Crassostrea gigas (C. gigas) were exposed to tralopyril (1 μg/L) and/or OA (PH = 7.7) for 21 days and a 14-day recovery acclimation. To investigate the stress response and potential molecular mechanisms of C. gigas to OA and tralopyril exposure alone or in combination, as well as the effects of OA and/or tralopyril on bivalve biomineralization and marine carbon cycling. The results showed that the combined toxicity was between that of acidification and tralopyril alone. Single or combined exposure activated the general stress defense responses of C. gigas mantle, affected energy metabolism and biomineralization of the organism and the carbon cycle of the marine ecosystem. Moreover, acidification-induced and tralopyril-induced toxicity showed potential recoverability at molecular and biochemical levels. This study provides a new perspective on the molecular mechanisms of tralopyril toxicity to bivalve shellfish and reveals the potential role of tralopyril and OA on marine carbon cycling.

Lipidomic insights into the immune response and pearl formation in transplanted pearl oyster Pinctada fucata martensii

During pearl culture, the excess immune responses may induce nucleus rejection and death of pearl oysters after transplantation. To better understand the immune response and pearl formation, lipidomic analysis was applied to investigate changes in the serum lipid profile of pearl oyster Pinctada fucata martensii following transplantation. In total, 296 lipid species were identified by absolute quantitation. During wound healing, the content of TG and DG initially increased and then decreased after 3 days of transplantation with no significant differences, while the level of C22:6 decreased significantly on days 1 and 3. In the early stages of transplantation, sphingosine was upregulated, whereas PC and PUFAs were downregulated in transplanted pearl oyster. PI was upregulated during pearl sac development stages. GP and LC-PUFA levels were upregulated during pearl formation stage. In order to identify enriched metabolic pathways, pathway enrichment analysis was conducted. Five metabolic pathways were found significantly enriched, namely glycosylphosphatidylinositol-anchor biosynthesis, glycerophospholipid metabolism, alpha-linolenic acid metabolism, linoleic acid metabolism and arachidonic acid metabolism. Herein, results suggested that the lipids involved in immune response, pearl sac maturation, and pearl formation in the host pearl oyster after transplantation, which might lead to an improvement in the survival rate and pearl quality of transplanted pearl oyster.

Organic Matrix and Secondary Metabolites in Nacre

Nacre, also called mother-of-pearl, is a naturally occurring biomineral, largely studied by chemists, structural biologists, and physicists to understand its outstanding and diverse properties. Nacre is constituted of aragonite nanograins surrounded by organic matrix, and it has been established that the organic matrix is responsible for initiating and guiding the biomineralization process. The first challenge to study the organic matrix of nacre lays in its separation from the biomineral. Several extraction methods have been developed so far. They are categorized as either strong (e.g., decalcification) or soft (e.g., water, ethanol) and they allow specific extractions of targeted compounds. The structure of the nacreous organic matrix is complex, and it provides interesting clues to describe the mineralization process. Proteins, sugars, lipids, peptides, and other molecules have been identified and their role in mineralization investigated. Moreover, the organic matrix of nacre has shown interesting properties for human health. Several studies are investigating its activity on bone mineralization and its properties for skin care. In this review, we focus on the organic constituents, as lipids, sugars, and small metabolites which are less studied since present in small quantities.

Proteomic and Transcriptomic Analyses in the Slipper Snail Crepidula fornicata Uncover Shell Matrix Genes Expressed During Adult and Larval Biomineralization

The gastropod shell is a composite composed of minerals and shell matrix proteins (SMPs). SMPs have been identified by proteomics in many molluscs, but few have been studied in detail. Open questions include (1) what gene regulatory networks regulate SMP expression, (2) what roles individual SMPs play in biomineralization, and (3) how the complement of SMPs changes over development. These questions are best addressed in a species in which gene perturbation studies are available; one such species is the slipper snail, Crepidula fornicata. Here, SEM and pXRD analysis demonstrated that the adult shell of C. fornicata exhibits crossed lamellar microstructure and is composed of aragonite. Using high-throughput proteomics we identified 185 SMPs occluded within the adult shell. Over half of the proteins in the shell proteome have known biomineralization domains, while at least 10% have no homologs in public databases. Differential gene expression analysis identified 20 SMP genes that are up-regulated in the shell-producing mantle tissue. Over half of these 20 SMPs are expressed during development with two, CfSMP1 and CfSMP2, expressed exclusively in the shell gland. Together, the description of the shell microstructure and a list of SMPs now sets the stage for studying the consequences of SMP gene knockdowns in molluscs.

DCSr-NL: A Novel Method to Semiquantitatively Probe the Growth Rate of Nacre

Matrix proteins play critical roles in regulating the prismatic and nacreous layer formation in the shell. However, due to the dearth of in vivo experiments, their specific roles during shell formation are still unclear. In this study, a new method to detect the content of Sr in the nacreous layer (DCSr-NL), which can semiquantitatively measure the nacreous growth rate, has been proposed. In vitro experiments show that during in vitro crystallization, the Sr element can replace Ca partially, resulting in isomorphism. In vivo experiments show that the best labeling conditions are when the Sr/Ca in seawater is 0.3, at 24 °C, and at 4 days of culture. Although a surface morphological difference in the inner layer of nacre is seldom detected by scanning electron microscopy (SEM), knockdown of the classical gene nacrein or unknown gene NU9, combined with DCSr-NL, shows that both significantly decrease the nacreous layer formation rate. The knockdown of the classical gene Pif177 or unknown genes NU3 or MRPN affects the surface morphology and decreases the nacreous layer formation rate. In general, thanks to DCSr-NL, we can efficiently analyze the growth rate of the nacre with or without morphological changes by SEM, and it is of considerable significance for exploring the target gene's function in forming the nacre in vivo.

Mantle tissue in the pearl oyster Pinctada fucata secretes immune components via vesicle transportation

Molluscan bivalves secrete shell matrices into the extrapallial space (EPS) to guide the precipitation of rigid shells. Meanwhile, immune components are present in the EPS and shell matrices, which are pivotal in resistant to invaded pathogens, thus ensuring the shell formation process. However, the origin of these components remains unclear. In this study, we revealed numerous vesicles were secreted from the outer mantle epithelial cells by using light and electron microscopes. The secreted vesicles were isolated by gradient centrifugation and confirmed by transmission electron microscopy. Proteomics analysis showed that the secreted vesicles were composed of cytoplasmic and immune components, most of which do not have signal peptides, indicating that they were secreted by a non-classical pathway. Moreover, real-time PCR revealed that some immune components were highly expressed in the mantle tissue, compared to the hemocytes. FTIR analysis verified the presence of lipids in the shell matrices, indicating that the vesicles have integrated into the shell layers. Taken together, our results suggested that mantle epithelial cells secreted some important immune components into the EPS via secreted vesicle transportation, thus cooperating with the hemocytes to play a vital role in immunity during shell formation.

Comparative Proteomics of Octocoral and Scleractinian Skeletomes and the Evolution of Coral Calcification

Corals are the ecosystem engineers of coral reefs, one of the most biodiverse marine ecosystems. The ability of corals to form reefs depends on the precipitation of calcium carbonate (CaCO₃) under biological control. However, several mechanisms underlying coral biomineralization remain elusive, for example, whether corals employ different molecular machineries to deposit different CaCO₃ polymorphs (i.e., aragonite or calcite). Here, we used tandem mass spectrometry (MS/MS) to compare the proteins occluded in the skeleton of three octocoral and one scleractinian species: Tubipora musica and Sinularia cf. cruciata (calcite sclerites), the blue coral Heliopora coerulea (aragonitic skeleton), and the scleractinian aragonitic Montipora digitata. Reciprocal Blast analysis revealed extremely low overlap between aragonitic and calcitic species, while a core set of proteins is shared between octocorals producing calcite sclerites. However, the carbonic anhydrase CruCA4 is present in the skeletons of both polymorphs. Phylogenetic analysis highlighted several possible instances of protein co-option in octocorals. These include acidic proteins and scleritin, which appear to have been secondarily recruited for calcification and likely derive from proteins playing different functions. Similarities between octocorals and scleractinians included presence of a galaxin-related protein, carbonic anhydrases, and one hephaestin-like protein. Although the first two appear to have been independently recruited, the third appear to share a common origin. This work represents the first attempt to identify and compare proteins associated with coral skeleton polymorph diversity, providing several new research targets and enabling both future functional and evolutionary studies aimed at elucidating the origin and evolution of coral biomineralization.

PU14, a Novel Matrix Protein, Participates in Pearl Oyster, Pinctada Fucata, Shell Formation

Biomineralization is a widespread biological process, involved in the formation of shells, teeth, and bones. Shell matrix proteins have been widely studied for their importance during shell formation. In 2015, our group identified 72 unique shell matrix proteins in Pinctada fucata, among which PU14 is a matrix protein detected in the soluble fraction that solely exists in the prismatic layer. However, the function of PU14 is still unclear. In this study, the full-length cDNA sequence of PU14 was obtained and functional analyses of PU14 protein during shell formation were performed. The deduced protein has a molecular mass of 77.8 kDa and an isoelectric point of 11.34. The primary protein structure contains Gln-rich and random repeat units, which are typical characteristics of matrix protein and indicate its potential function during shell formation. In vivo and in vitro experiments indicated PU14 has prismatic layer functions during shell formation. The tissue expression patterns showed that PU14 was mainly expressed in the mantle tissue, which is consistent with prismatic layer formation. Notching experiments suggested that PU14 responded to repair and regenerate the injured shell. After inhibiting gene expression by injecting PU14-specific double-stranded RNA, the inner surface of the prismatic layer changed significantly and became rougher. Further, in vitro experiments showed that recombinant protein rPU14 impacted calcite crystal morphology. Taken together, characterization and functional analyses of a novel matrix protein, PU14, provide new insights about basic matrix proteins and their functions during shell formation.

Molecular Pathways and Pigments Underlying the Colors of the Pearl Oyster Pinctada margaritifera var. cumingii (Linnaeus 1758)

The shell color of the Mollusca has attracted naturalists and collectors for hundreds of years, while the molecular pathways regulating pigment production and the pigments themselves remain poorly described. In this study, our aim was to identify the main pigments and their molecular pathways in the pearl oyster Pinctada margaritifera—the species displaying the broadest range of colors. Three inner shell colors were investigated—red, yellow, and green. To maximize phenotypic homogeneity, a controlled population approach combined with common garden conditioning was used. Comparative analysis of transcriptomes (RNA-seq) of P. margaritifera with different shell colors revealed the central role of the heme pathway, which is involved in the production of red (uroporphyrin and derivates), yellow (bilirubin), and green (biliverdin and cobalamin forms) pigments. In addition, the Raper–Mason, and purine metabolism pathways were shown to produce yellow pigments (pheomelanin and xanthine) and the black pigment eumelanin. The presence of these pigments in pigmented shell was validated by Raman spectroscopy. This method also highlighted that all the identified pathways and pigments are expressed ubiquitously and that the dominant color of the shell is due to the preferential expression of one pathway compared with another. These pathways could likely be extrapolated to many other organisms presenting broad chromatic variation.

Integrated GC-MS- and LC-MS-Based Untargeted Metabolomics Studies of the Effect of Vitamin D3 on Pearl Production Traits in Pearl Oyster Pinctada fucata martensii

Pearl oyster Pinctada fucata martensii is widely recognized for biomineralization and has been cultured for high-quality marine pearl production. To ascertain how dietary vitamin D3 (VD3) levels affect the features of pearl production by P. f. martensii and discover the mechanisms regulating this occurrence, five experimental diets with variable levels of VD3 were used with inclusion levels of 0, 500, 1,000, 3,000, and 10,000 IU/kg. The distinct inclusion levels were distributed into five experimental groups (EG1, EG2, EG3, EG4, and EG5). All the experimental groups were reared indoors except the control group (CG) reared at the sea. Pearl oysters, one year and a half old, were used in the grafting operation to culture pearls. During the growing period that lasted 137 days, EG3 had the highest survival rate, retention rate, and high-quality pearl rate. A similar trend was found for EG3 and CG with significantly higher pearl thickness and nacre deposition rates than other groups, but no significant differences were observed between them. A metabolomics profiling using GC–MS and LC–MS of pearl oysters fed with low quantities of dietary VD3 and optimal levels of dietary VD3 revealed 135 statistically differential metabolites (SDMs) (VIP > 1 and p < 0.05). Pathway analysis indicated that SDMs were involved in 32 pathways, such as phenylalanine metabolism, histidine metabolism, glycerophospholipid metabolism, alanine aspartate and glutamate metabolism, arginine and proline metabolism, glycerolipid metabolism, amino sugar and nucleotide sugar metabolism, and tyrosine metabolism. These results provide a theoretical foundation for understanding the impacts of VD3 on pearl production traits in pearl oyster and reinforce forthcoming prospects and application of VD3 in pearl oyster in aquaculture rearing conditions.

Cloning, characterization, and functional analysis of chitinase-like protein 1 in the shell of Pinctada fucata

Biomineralization, especially shell formation, is a sophisticated process regulated by various matrix proteins. Pinctada fucata chitinase-like protein 1 (Pf-Clp1), which belongs to the GH18 family, was discovered by our group using in-depth proteomic analysis. However, its function is still unclear. In this study, we first obtained the full-length cDNA sequence of Pf-Clp1 by RACE. Real-time polymerase chain reaction results revealed that Pf-Clp1 was highly expressed in the important biomineralization tissues, the mantle edge and the mantle pallial. We expressed and purified recombinant protein rPf-Clp1 in vitro to investigate the function of Pf-Clp1 on CaCO3 crystallization. Scanning electron microscopy imaging and Raman spectroscopy revealed that rPf-Clp1 was able to affect the morphologies of calcite crystal in vitro. Shell notching experiments suggested that Pf-Clp1 might function as a negative regulator during shell formation in vivo. Knockdown of Pf-Clp1 by RNAi led to the overgrowth of aragonite tablets, further confirming its potential negative regulation on biomineralization, especially in the nacreous layer. Our work revealed the potential function of molluscan Clp in shell biomineralization for the first time and unveiled some new understandings toward the molecular mechanism of shell formation.

Trivalent Iron Is Responsible for the Yellow Color Development in the Nacre of Akoya Pearl Oyster Shells

The gold and cream colors of cultured Akoya pearls, as well as natural yellow nacre of pearl oyster shells, are thought to arise from intrinsic yellow pigments. While the isolation of the yellow pigments has been attempted using a large amount of gold pearls, the substance concerned is still unknown. We report here on the purification and characterization of yellow pigments from the nacre of Akoya pearl oyster shells. Two yellow components, YC1 and YC2, were isolated from the HCl-methanol (HCl-MeOH) extract from nacreous organic matrices obtained by decalcification of the shells with ethylenediaminetetraacetic acid (EDTA). Energy-dispersive X-ray and infrared spectroscopy analyses suggested that YC1 and YC2 precipitated under basic conditions are composed of Fe-containing inorganic and polyamide-containing organic compounds, respectively. YC1 solubilized under acidic conditions exhibited positive reactions to KSCN and K₄[Fe(CN)₆] reagents, showing the same ultraviolet-visible absorption spectrum as those of Fe(III)-containing compounds. In addition, X-ray absorption fine structure analysis supported the compound in the form of Fe(III). The total amount of Fe was approximately 2.6 times higher in the yellow than white nacre, and most Fe was fractionated into the EDTA-decalcifying and HCl-MeOH extracts. These results suggest that Fe(III) coordinated to EDTA-soluble and insoluble matrix compounds are mainly associated with yellow color development not only in the Akoya pearl oyster shells but also in the cultured Akoya pearls.

Extraction techniques and potential health benefits of bioactive compounds from marine molluscs: a review

Marine molluscs and their bioactive compounds are of particular relevance to the growing pool of nutraceutical resources under global investigation.

The Prismatic Layer of Pinna: A Showcase of Methodological Problems and Preconceived Hypotheses

The prismatic layer of Pinna (Mollusca) is one of the most studied models for the understanding of the biomineralization mechanisms, but our knowledge of the organic components of this layer is limited to the proteins of the soluble organic matrices. The interplay of the mineral and organic matrices is studied using scanning electron and atomic force microscopy, infra-red spectrometry, thermogravimetric analyses, aminoacids analyses, thin layer chromatography (TLC), X-ray fluorescence, X-ray Absorption near Edge Structure (XANES) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). Proteins, polysaccharides and lipids are detected within the prisms and their envelopes. The role of the technical choices to study calcareous biominerals is evidenced, showing that a single analysis is not enough to decipher complex biominerals.

Cloning, characterization and functional analysis of an Alveoline-like protein in the shell of Pinctada fucata

Shell matrix proteins (SMPs) have important functions in biomineralization. In the past decades, the roles of SMPs were gradually revealed. In 2015, our group identified 72 unique SMPs in Pinctada fucata, among which Alveoline-like (Alv) protein was reported to have homologous genes in Pinctada maxima and Pinctada margaritifera. In this study, the full-length cDNA sequence of Alv and the functional analysis of Alv protein during shell formation were explored. The deduced protein (Alv), which has a molecular mass of 24.9 kDa and an isoelectric point of 11.34, was characterized, and the functional analyses was explored in vivo and in vitro. The Alv gene has high expression in mantle and could response to notching damage. The functional inhibition of Alv protein in vivo by injecting recombinant Alv (rAlv) antibodies destroyed prism structure but accelerated nacre growth. Western blot and immunofluorescence staining showed that native Alv exists in the EDTA-insoluble matrix of both prismatic and nacreous layers and has different distribution patterns in the inner or outer prismatic layer. Taken together, the characterization and functional analyses of matrix protein Alv could expand our understanding of basic matrix proteins and their functions during shell formation.

Molecular and functional analysis of PmCHST1b in nacre formation of Pinctada fucata martensii

Keratan sulfate possesses considerable amounts of negatively charged sulfonic acid groups and participates in biomineralization. In the present study, we investigated characteristics and functions of a CHST1 gene identified from the pearl oyster Pinctada fucata martensii (PmCHST1b) which participated in the synthesis of keratan sulfate. PmCHST1b amino acid sequence carried a typical sulfotransferase-3 domain (sulfotransfer-3 domain) and belonged to membrane-associated sulfotransferases. Homologous analysis of CHST1 from different species showed the conserved motif (5' PSB motif and 3' PB motif) which interacted with 3'-phosphoadenosine-5'-phosphosulfate (PAPS). Structure analysis of sulfotransferase domain indicted that PmCHST1b showed the conserved catalytic structure character and the relationships presented in the phylogenetic tree conformed to that of traditional taxonomy. Expression pattern of PmCHST1b in different tissues and development stages showed that PmCHST1b widely expressed in all the detected tissues and development stages and showed the highest expression level in the central zone of mantle (MC). PmCHST1b expressed highly in the trochophore, D-stage larvae and spat which corresponded to prodissoconch and dissoconch shell formation, respectively. RNA interference (RNAi) successfully inhibited expression level of PmCHST1b in MC (P<0.05), and sulfate polymer content in the extrapallial fluid significantly reduced (P<0.05). Crystallization of shell nacre became irregular. Results above indicated that PmCHST1b may affect nacre formation by participating in synthesis of keratan sulfate in extrapallial fluid. This study provided fundamental materials for further research on the role of sulfotransferases and keratan sulfate in nacre formation.

Molecular modularity and asymmetry of the molluscan mantle revealed by a gene expression atlas

Background

Conchiferan molluscs construct a biocalcified shell that likely supported much of their evolutionary success. However, beyond broad proteomic and transcriptomic surveys of molluscan shells and the shell-forming mantle tissue, little is known of the spatial and ontogenetic regulation of shell fabrication. In addition, most efforts have been focused on species that deposit nacre, which is at odds with the majority of conchiferan species that fabricate shells using a crossed-lamellar microstructure, sensu lato.

Results

By combining proteomic and transcriptomic sequencing with in situ hybridization we have identified a suite of gene products associated with the production of the crossed-lamellar shell in Lymnaea stagnalis. With this spatial expression data we are able to generate novel hypotheses of how the adult mantle tissue coordinates the deposition of the calcified shell. These hypotheses include functional roles for unusual and otherwise difficult-to-study proteins such as those containing repetitive low-complexity domains. The spatial expression readouts of shell-forming genes also reveal cryptic patterns of asymmetry and modularity in the shell-forming cells of larvae and adult mantle tissue.

Conclusions

This molecular modularity of the shell-forming mantle tissue hints at intimate associations between structure, function, and evolvability and may provide an elegant explanation for the evolutionary success of the second largest phylum among the Metazoa.

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.

A Novel Matrix Protein, PfY2, Functions as a Crucial Macromolecule during Shell Formation

Biomineralization, including shell formation, is dedicatedly regulated by matrix proteins. PfY2, a matrix protein detected in the ethylene diamine tetraacetic acid (EDTA)-soluble fraction from both prismatic layer and nacreous layer, was discovered by our group using microarray. It may play dual roles during biomineralization. However, the molecular mechanism is still unclear. In this research, we studied the function of PfY2 on crystallization in vivo and in vitro, revealing that it might be a negative regulator during shell formation. Notching experiment indicated that PfY2 was involved in shell repairing and regenerating process. Repression of PfY2 gene affected the structure of prismatic and nacreous layer simultaneously, confirming its dual roles in shell formation. Recombinant protein rPfY2 significantly suppressed CaCO₃ precipitation rate, participated in the crystal nucleation process, changed the morphology of crystals and inhibited the transformation of amorphous calcium carbonate (ACC) to stable calcite or aragonite in vitro. Our results may provide new evidence on the biomineralization inhibition process.

Ecological relevance of skeletal fatty acid concentration and composition in Mediterranean scleractinian corals

The intra-skeletal fatty acid concentration and composition of four Mediterranean coral species, namely Cladocora caespitosa, Balanophyllia europaea, Astroides calycularis and Leptopsammia pruvoti, were examined in young and old individuals living in three different locations of the Mediterranean Sea. These species are characterized by diverse levels of organization (solitary or colonial) and trophic strategies (symbiotic or non-symbiotic). Fatty acids have manifold fundamental roles comprehensive of membrane structure fluidity, cell signaling and energy storage. For all species, except for B. europaea, the intra-skeletal fatty acid concentration was significantly higher in young individuals than in old ones. Moreover, fatty acid concentration was higher in colonial corals than in solitary ones and in the symbiotic corals compared to non-symbiotic ones. Analysis by gas chromatography-mass spectrometry (GC-MS) revealed that palmitic acid (16:0) was the most abundant fatty acid, followed by stearic (18:0) in order of concentration. Oleic acid (18:1) was detected as the third main component only in skeletons from symbiotic corals. These results suggest that, in the limits of the studied species, intra-skeletal fatty acid composition and concentration may be used for specific cases as a proxy of level of organization and trophic strategy, and eventually coral age.

Identification and Characterization of the Lysine-Rich Matrix Protein Family in Pinctada fucata: Indicative of Roles in Shell Formation

Mantle can secret matrix proteins playing key roles in regulating the process of shell formation. The genes encoding lysine-rich matrix proteins (KRMPs) are one of the most highly expressed matrix genes in pearl oysters. However, the expression pattern of KRMPs is limited and the functions of them still remain unknown. In this study, we isolated and identified six new members of lysine-rich matrix proteins, rich in lysine, glycine and tyrosine, and all of them are basic matrix proteins. Combined with four members of the KRMPs previously reported, all these proteins can be divided into three subclasses according to the results of phylogenetic analyses: KRMP1-3 belong to subclass KPI, KRMP4-5 belong to KPII, and KRMP6-10 belong to KPIII. Three subcategories of lysine-rich matrix proteins are highly expressed in the D-phase, the larvae and adult mantle. Lysine-rich matrix proteins are involved in the shell repairing process and associated with the formation of the shell and pearl. What's more, they can cause abnormal shell growth after RNA interference. In detail, KPI subgroup was critical for the beginning formation of the prismatic layer; both KPII and KPIII subgroups participated in the formation of prismatic layer and nacreous layer. Compared with different temperatures and salinity stimulation treatments, the influence of changes in pH on KRMPs gene expression was the greatest. Recombinant KRMP7 significantly inhibited CaCO₃ precipitation, changed the morphology of calcite, and inhibited the growth of aragonite in vitro. Our results are beneficial to understand the functions of the KRMP genes during shell formation.

Dual Roles of the Lysine-Rich Matrix Protein (KRMP)-3 in Shell Formation of Pearl Oyster, Pinctada fucata

Matrix proteins play important roles in shell formation. Our group firstly isolated three cDNAs encoding lysine-rich matrix protein from Pinctada fucata in 2006. However, the functions of KRMPs are not fully understood. In addition, KRMPs contain two functional domains, the basic domain and the Gly/Tyr domain respectively. Based on the modular organization, the roles of their two domains were poorly characterized. Furthermore, KRMPs were then reported in other two species, P. maxima and P. margaritifera, which indicated that KRMPs might be very important for shell formation. In this study, the characterization and function of KRMP-3 and its two functional domains were studied in vitro through purification of recombinant glutathione S-transferase tagged KRMP-3 and two KRMP-3 deletion mutants. Western blot and immunofluorescence revealed that native KRMP-3 existed in the EDTA-insoluble matrix of the prismatic layer and was located in the organic sheet and the prismatic sheath. Recombinant KRMP-3 (rKRMP-3) bound tightly to chitin and this binding capacity was duo to the Gly/Tyr-rich region. rKRMP-3 inhibited the precipitation of CaCO₃, affected the crystal morphology of calcite and inhibited the growth of aragonite in vitro, which was almost entirely attributed to the lysine-rich region. The results present direct evidence of the roles of KRMP-3 in shell biomineralization. The functional rBR region was found to participate in the growth control of crystals and the rGYR region was responsible to bind to chitin.

Ultrastructure of the Interlamellar Membranes of the Nacre of the Bivalve Pteria hirundo, Determined by Immunolabelling

The current model for the ultrastructure of the interlamellar membranes of molluscan nacre imply that they consist of a core of aligned chitin fibers surrounded on both sides by acidic proteins. This model was based on observations taken on previously demineralized shells, where the original structure had disappeared. Despite other earlier claims, no direct observations exist in which the different components can be unequivocally discriminated. We have applied different labeling protocols on non-demineralized nacreous shells of the bivalve Pteria. With this method, we have revealed the disposition and nature of the different fibers of the interlamellar membranes that can be observed on the surface of the nacreous shell of the bivalve Pteria hirundo by high resolution scanning electron microscopy (SEM). The minor chitin component consists of very thin fibers with a high aspect ratio and which are seemingly disoriented. Each fiber has a protein coat, which probably forms a complex with the chitin. The chitin-protein-complex fibers are embedded in an additional proteinaceous matrix. This is the first time in which the sizes, positions and distribution of the chitin fibers have been observed in situ.

Mollusk shell structures and their formation mechanism

In nature, mollusk shells have a role in protecting the soft body of the mollusk from predators and from the external environment, and the shells consist mainly of calcium carbonate and small amounts of organic matrices. Organic matrices in mollusk shells are thought to play key roles in shell formation. However, enough information has not been accumulated so far. High toughness and stiffness have been focused on as being adaptable to the development of organic–inorganic hybrid materials. Because mollusks can produce elaborate microstructures containing organic matrices under ambient conditions, the investigation of shell formation is expected to lead to the development of new inorganic–organic hybrid materials for various applications. In this review paper, we summarize the structures of mollusk shells and their process of formation, together with the analysis of various organic matrices related to shell calcification.

A therapeutic potential for marine skeletal proteins in bone regeneration

A vital ingredient for engineering bone tissue, in the culture dish, is the use of recombinant matrix and growth proteins to help accelerate the growth of cultivated tissues into clinically acceptable quantities. The skeletal organic matrices of calcifying marine invertebrates are an untouched potential source of such growth inducing proteins. They have the advantage of being ready-made and retain the native state of the original protein. Striking evidence shows that skeleton building bone morphogenic protein-2/4 (BMP) and transforming growth factor beta (TGF-β) exist within various marine invertebrates such as, corals. Best practice mariculture and the latest innovations in long-term marine invertebrate cell cultivation can be implemented to ensure that these proteins are produced sustainably and supplied continuously. This also guarantees that coral reef habitats are not damaged during the collection of specimens. Potential proteins for bone repair, either extracted from the skeleton or derived from cultivated tissues, can be identified, evaluated and retrieved using chromatography, cell assays and proteomic methods. Due to the current evidence for bone matrix protein analogues in marine invertebrates, together with the methods established for their production and retrieval there is a genuine prospect that they can be used to regenerate living bone for potential clinical use.