An Antarctic molluscan biomineralisation tool-kit

Diverting the Use of Hand-Operated Tablet Press Machines to Bioassays: A Novel Protocol to Test 'Waste' Insoluble Shell Matrices

To mineralize their shells, molluscs secrete a complex cocktail of proteins-collectively defined as the calcifying shell matrix-that remains occluded in the exoskeleton. Nowadays, protein extracts from shells are recognized as a potential source of bioactive substances, among which signalling molecules, bactericides or protease inhibitors offer the most tangible perspectives in applied sciences, health, and aquaculture. However, one technical obstacle in testing the activity of shell extracts lies in their high insolubility. In this paper, we present a protocol that circumvents this impediment. After an adapted shell protein extraction and the production of two organic fractions-one soluble, one insoluble-we employ a hand-operated tablet press machine to generate well-calibrated tablets composed of 100% insoluble shell matrix. FT-IR monitoring of the quality of the tablets shows that the pressure used in the press machine does not impair the molecular properties of the insoluble extracts. The produced tablets can be directly tested in different biological assays, such as the bactericidal inhibition zone assay in Petri dish, as illustrated here. Diverting the use of the hand-operated tablet press opens new perspectives in the analysis of insoluble shell matrices, for discovering novel bioactive components.

Cell type and gene regulatory network approaches in the evolution of spiralian biomineralisation

Biomineralisation is the process by which living organisms produce hard structures such as shells and bone. There are multiple independent origins of biomineralised skeletons across the tree of life. This review gives a glimpse into the diversity of spiralian biominerals and what they can teach us about the evolution of novelty. It discusses different levels of biological organisation that may be informative to understand the evolution of biomineralisation and considers the relationship between skeletal and non-skeletal biominerals. More specifically, this review explores if cell type and gene regulatory network approaches could enhance our understanding of the evolutionary origins of biomineralisation.

Evolutionary conservation and divergence of the transcriptional regulation of bivalve shell secretion across life-history stages

Adult molluscs produce shells with diverse morphologies and ornamentations, different colour patterns and microstructures. The larval shell, however, is a phenotypically more conserved structure. How do developmental and evolutionary processes generate varying diversity at different life-history stages within a species? Using live imaging, histology, scanning electron microscopy and transcriptomic profiling, we have described shell development in a heteroconchian bivalve, the Antarctic clam, Laternula elliptica, and compared it to adult shell secretion processes in the same species. Adult downstream shell genes, such as those encoding extracellular matrix proteins and biomineralization enzymes, were largely not expressed during shell development. Instead, a development-specific downstream gene repertoire was expressed. Upstream regulatory genes such as transcription factors and signalling molecules were largely conserved between developmental and adult shell secretion. Comparing heteroconchian data with recently reported pteriomorphian larval shell development data suggests that, despite being phenotypically more conserved, the downstream effectors constituting the larval shell 'tool-kit' may be as diverse as that of adults. Overall, our new data suggest that a larval shell formed using development-specific downstream effector genes is a conserved and ancestral feature of the bivalve lineage, and possibly more broadly across the molluscs.

Assessment of the juvenile vulnerability of symbiont-bearing giant clams to ocean acidification

Ocean acidification (OA) severely affects marine bivalves, especially their calcification processes. However, very little is known about the fate of symbiont-bearing giant clams in the acidified oceans, which hinders our ability to develop strategies to protect this ecologically and economically important group in coral reef ecosystems. Here, we explored the integrated juvenile responses of fluted giant clam Tridacna squamosa (Lamarck, 1819) to acidified seawater at different levels of biological organization. Our results revealed that OA did not cause a significant reduction in survival and shell growth performance, indicating that T. squamosa juveniles are tolerated to moderate acidification. Yet, significantly reduced net calcification rate demonstrated the calcifying physiology sensitivity to OA, in line with significant declines in symbiont photosynthetic yield and zooxanthellae density which in turn lowered the amount of energy supply for energetically expensive calcification processes. Subsequent transcriptome sequencing and comparative analysis of differentially expressed genes revealed that the regulation of calcification processes, such as transport of calcification substrates, acid-base regulation, synthesis of organic matrix in the calcifying fluid, as well as metabolic depression were the major response to OA. Taken together, the integration of physiological and molecular responses can provide a comprehensive understanding of how the early life history stages of giant clams respond to OA and make an important leap forward in assessing their fate under future ocean conditions.

The degradation of intracrystalline mollusc shell proteins: A proteomics study of Spondylus gaederopus

Mollusc shells represent excellent systems for the preservation and retrieval of genuine biomolecules from archaeological or palaeontological samples. As a consequence, the post-mortem breakdown of intracrystalline mollusc shell proteins has been extensively investigated, particularly with regard to its potential use as a "molecular clock" for geochronological applications. But despite seventy years of ancient protein research, the fundamental aspects of diagenesis-induced changes to protein structures and sequences remain elusive. In this study we investigate the degradation of intracrystalline proteins by performing artificial degradation experiments on the shell of the thorny oyster, Spondylus gaederopus, which is particularly important for archaeological research. We used immunochemistry and tandem mass tag (TMT) quantitative proteomics to simultaneously track patterns of structural loss and of peptide bond hydrolysis. Powdered and bleached shell samples were heated in water at four different temperatures (80, 95, 110, 140 °C) for different time durations. The structural loss of carbohydrate and protein groups was investigated by immunochemical techniques (ELLA and ELISA) and peptide bond hydrolysis was studied by tracking the changes in protein/peptide relative abundances over time using TMT quantitative proteomics. We find that heating does not induce instant organic matrix decay, but first facilitates the uncoiling of cross-linked structures, thus improving matrix detection. We calculated apparent activation energies of structural loss: E_a (carbohydrate groups) = 104.7 kJ/mol, E_a (protein epitopes) = 104.4 kJ/mol, which suggests that secondary matrix structure degradation may proceed simultaneously with protein hydrolysis. While prolonged heating at 110 °C (10 days) results in complete loss of the structural signal, surviving peptide sequences were still observed. Eight hydrolysis-prone peptide bonds were identified in the top scoring shell sequence, the uncharacterised protein LOC117318053 from Pecten maximus. Interestingly, these were not the expected "weak" bonds based on published theoretical stabilities calculated for peptides in solution. This further confirms that intracrystalline protein degradation patterns are complex and that the overall microchemical environment plays an active role in protein stability. Our TMT approach represents a major stepping stone towards developing a model for studying protein diagenesis in biomineralised systems.

A bivalve biomineralization toolbox

Mollusc shells are a result of the deposition of crystalline and amorphous calcite catalysed by enzymes and shell matrix proteins. Developing a detailed understanding of bivalve mollusc biomineralization pathways is complicated not only by the multiplicity of shell forms and microstructures in this class, but also by the evolution of associated proteins by domain co-option and domain shuffling. In spite of this, a minimal biomineralization toolbox comprising proteins and protein domains critical for shell production across species has been identified. Using a matched pair design to reduce experimental noise from inter-individual variation, combined with damage-repair experiments and a database of biomineralization shell matrix proteins (SMP) derived from published works, proteins were identified that are likely to be involved in shell calcification. Eighteen new, shared proteins likely to be involved in the processes related to the calcification of shells were identified by analysis of genes expressed during repair in Crassostrea gigas, Mytilus edulis and Pecten maximus. Genes involved in ion transport were also identified as potentially involved in calcification either via the maintenance of cell acid-base balance or transport of critical ions to the extrapallial space, the site of shell assembly. These data expand the number of candidate biomineralization proteins in bivalve molluscs for future functional studies and define a minimal functional protein domain set required to produce solid microstructures from soluble calcium carbonate. This is important for understanding molluscan shell evolution, the likely impacts of environmental change on biomineralization processes, materials science, and biomimicry research.

Biomineral proteomics: A tool for multiple disciplinary studies

The hard tissues of animals, such as skeletons and teeth, are constructed by a biologically controlled process called biomineralization. In invertebrate animals, biominerals are considered important for their evolutionary success. These biominerals are hieratical biocomposites with excellent mechanical properties, and their formation has intrigued researchers for decades. Although proteins account for ~5 wt% of biominerals, they are critical players in biomineralization. With the development of high-throughput analysis methods, such as proteomics, biomineral protein data are rapidly accumulating, thus necessitating a refined model for biomineralization. This review focuses on biomineral proteomics in invertebrate animals to highlight the diversity of biomineral proteins (generally 40-80 proteins), and the results indicate that biomineralization includes thermodynamic crystal growth as well as intense extracellular matrix activity and/or vesicle transport. Biominerals have multiple functions linked to biological immunity and antipathogen activity. A comparison of proteomes across species and biomineral types showed that von Willebrand factor type A and epidermal growth factor, which frequently couple with other extracellular domains, are the most common domains. Combined with species-specific repetitive low complexity domains, shell matrix proteins can be employed to predict biomineral types. Furthermore, this review discusses the applications of biomineral proteomics in diverse fields, such as tissue regeneration, developmental biology, archeology, environmental science, and material science.

Computationally predicted gene regulatory networks in molluscan biomineralization identify extracellular matrix production and ion transportation pathways

MOTIVATION

The molecular processes regulating molluscan shell production remain relatively uncharacterized, despite the clear evolutionary and societal importance of biomineralization.

RESULTS

Here we built the first computationally predicted gene regulatory network (GRN) for molluscan biomineralization using Antarctic clam (Laternula elliptica) mantle gene expression data produced over an age-categorized shell damage-repair time-course. We used previously published in vivo in situ hybridization expression data to ground truth gene interactions predicted by the GRN and show that candidate biomineralization genes from different shell layers, and hence microstructures, were connected in unique modules. We characterized two biomineralization modules of the GRN and hypothesize that one module is responsible for translating the extracellular proteins required for growing, repairing or remodelling the nacreous shell layer, whereas the second module orchestrates the transport of both ions and proteins to the shell secretion site, which are required during normal shell growth, and repair. Our findings demonstrate that unbiased computational methods are particularly valuable for studying fundamental biological processes and gene interactions in non-model species where rich sources of gene expression data exist, but annotation rates are poor and the ability to carry out true functional tests are still lacking.

AVAILABILITY AND IMPLEMENTATION

The raw RNA-Seq data is freely available for download from NCBI SRA (Accession: PRJNA398984), the assembled and annotated transcriptome can be viewed and downloaded from molluscDB (ensembl.molluscdb.org) and in addition, the assembled transcripts, reconstructed GRN, modules and detailed annotations are all available as Supplementary Files.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Deciphering mollusc shell production: the roles of genetic mechanisms through to ecology, aquaculture and biomimetics

Most molluscs possess shells, constructed from a vast array of microstructures and architectures. The fully formed shell is composed of calcite or aragonite. These CaCO₃ crystals form complex biocomposites with proteins, which although typically less than 5% of total shell mass, play significant roles in determining shell microstructure. Despite much research effort, large knowledge gaps remain in how molluscs construct and maintain their shells, and how they produce such a great diversity of forms. Here we synthesize results on how shell shape, microstructure, composition and organic content vary among, and within, species in response to numerous biotic and abiotic factors. At the local level, temperature, food supply and predation cues significantly affect shell morphology, whilst salinity has a much stronger influence across latitudes. Moreover, we emphasize how advances in genomic technologies [e.g. restriction site-associated DNA sequencing (RAD-Seq) and epigenetics] allow detailed examinations of whether morphological changes result from phenotypic plasticity or genetic adaptation, or a combination of these. RAD-Seq has already identified single nucleotide polymorphisms associated with temperature and aquaculture practices, whilst epigenetic processes have been shown significantly to modify shell construction to local conditions in, for example, Antarctica and New Zealand. We also synthesize results on the costs of shell construction and explore how these affect energetic trade-offs in animal metabolism. The cellular costs are still debated, with CaCO₃ precipitation estimates ranging from 1-2 J/mg to 17-55 J/mg depending on experimental and environmental conditions. However, organic components are more expensive (~29 J/mg) and recent data indicate transmembrane calcium ion transporters can involve considerable costs. This review emphasizes the role that molecular analyses have played in demonstrating multiple evolutionary origins of biomineralization genes. Although these are characterized by lineage-specific proteins and unique combinations of co-opted genes, a small set of protein domains have been identified as a conserved biomineralization tool box. We further highlight the use of sequence data sets in providing candidate genes for in situ localization and protein function studies. The former has elucidated gene expression modularity in mantle tissue, improving understanding of the diversity of shell morphology synthesis. RNA interference (RNAi) and clustered regularly interspersed short palindromic repeats - CRISPR-associated protein 9 (CRISPR-Cas9) experiments have provided proof of concept for use in the functional investigation of mollusc gene sequences, showing for example that Pif (aragonite-binding) protein plays a significant role in structured nacre crystal growth and that the Lsdia1 gene sets shell chirality in Lymnaea stagnalis. Much research has focused on the impacts of ocean acidification on molluscs. Initial studies were predominantly pessimistic for future molluscan biodiversity. However, more sophisticated experiments incorporating selective breeding and multiple generations are identifying subtle effects and that variability within mollusc genomes has potential for adaption to future conditions. Furthermore, we highlight recent historical studies based on museum collections that demonstrate a greater resilience of molluscs to climate change compared with experimental data. The future of mollusc research lies not solely with ecological investigations into biodiversity, and this review synthesizes knowledge across disciplines to understand biomineralization. It spans research ranging from evolution and development, through predictions of biodiversity prospects and future-proofing of aquaculture to identifying new biomimetic opportunities and societal benefits from recycling shell products.

Molecular mechanisms of biomineralization in marine invertebrates

Much recent marine research has been directed towards understanding the effects of anthropogenic-induced environmental change on marine biodiversity, particularly for those animals with heavily calcified exoskeletons, such as corals, molluscs and urchins. This is because life in our oceans is becoming more challenging for these animals with changes in temperature, pH and salinity. In the future, it will be more energetically expensive to make marine skeletons and the increasingly corrosive conditions in seawater are expected to result in the dissolution of these external skeletons. However, initial predictions of wide-scale sensitivity are changing as we understand more about the mechanisms underpinning skeletal production (biomineralization). These studies demonstrate the complexity of calcification pathways and the cellular responses of animals to these altered conditions. Factors including parental conditioning, phenotypic plasticity and epigenetics can significantly impact the production of skeletons and thus future population success. This understanding is paralleled by an increase in our knowledge of the genes and proteins involved in biomineralization, particularly in some phyla, such as urchins, molluscs and corals. This Review will provide a broad overview of our current understanding of the factors affecting skeletal production in marine invertebrates. It will focus on the molecular mechanisms underpinning biomineralization and how knowledge of these processes affects experimental design and our ability to predict responses to climate change. Understanding marine biomineralization has many tangible benefits in our changing world, including improvements in conservation and aquaculture and exploitation of natural calcified structure design using biomimicry approaches that are aimed at producing novel biocomposites.

The shell matrix of the european thorny oyster, Spondylus gaederopus: microstructural and molecular characterization

Molluscs, the largest marine phylum, display extraordinary shell diversity and sophisticated biomineral architectures. However, mineral-associated biomolecules involved in biomineralization are still poorly characterised. We report the first comprehensive structural and biomolecular study of Spondylus gaederopus, a pectinoid bivalve with a peculiar shell texture. Used since prehistoric times, this is the best-known shell of Europe's cultural heritage. We find that Spondylus microstructure is very poor in mineral-bound organics, which are mostly intercrystalline and concentrated at the interface between structural layers. Using high-resolution liquid chromatography tandem mass spectrometry (LC-MS/MS) we characterized several shell protein fractions, isolated following different bleaching treatments. Several peptides were identified as well as six shell proteins, which display features and domains typically found in biomineralized tissues, including the prevalence of intrinsically disordered regions. It is very likely that these sequences only partially represent the full proteome of Spondylus, considering the lack of genomics data for this genus and the fact that most of the reconstructed peptides do not match with any known shell proteins, representing consequently lineage-specific sequences. This work sheds light onto the shell matrix involved in the biomineralization in spondylids. Our proteomics data suggest that Spondylus has evolved a shell-forming toolkit, distinct from that of other better studied pectinoids - fine-tuned to produce shell structures with high mechanical properties, while limited in organic content. This study therefore represents an important milestone for future studies on biomineralized skeletons and provides the first reference dataset for forthcoming molecular studies of Spondylus archaeological artifacts.

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.