Insights into Mollusk Shell Formation: Interlamellar and Lamellar-Specific Nacre Protein Hydrogels Differ in Ion Interaction Signatures

Molecular adaptation of molluscan biomineralisation to high-CO2 oceans - The known and the unknown

High-CO₂ induced ocean acidification (OA) reduces the calcium carbonate (CaCO₃) saturation level (Ω) and the pH of oceans. Consequently, OA is causing a serious threat to several ecologically and economically important biomineralising molluscs. Biomineralisation is a highly controlled biochemical process by which molluscs deposit their calcareous structures. In this process, shell matrix proteins aid the nucleation, growth and assemblage of the CaCO₃ crystals in the shell. These molluscan shell proteins (MSPs) are, ultimately, responsible for determination of the diverse shell microstructures and mechanical strength. Recent studies have attempted to integrate gene and protein expression data of MSPs with shell structure and mechanical properties. These advances made in understanding the molecular mechanism of biomineralisation suggest that molluscs either succumb or adapt to OA stress. In this review, we discuss the fate of biomineralisation process in future high-CO₂ oceans and its ultimate impact on the mineralised shell's structure and mechanical properties from the perspectives of limited substrate availability theory, proton flux limitation model and the omega myth theory. Furthermore, studying the interplay of energy availability and differential gene expression is an essential first step towards understanding adaptation of molluscan biomineralisation to OA, because if there is a need to change gene expression under stressors, any living system would require more energy than usual. To conclude, we have listed, four important future research directions for molecular adaptation of molluscan biomineralisation in high-CO₂ oceans: 1) Including an energy budgeting factor while understanding differential gene expression of MSPs and ion transporters under OA. 2) Unraveling the genetic or epigenetic changes related to biomineralisation under stressors to help solving a bigger picture about future evolution of molluscs, and 3) Understanding Post Translational Modifications of MSPs with and without stressors. 4) Understanding carbon uptake mechanisms across taxa with and without OA to clarify the OA theories on Ω.

A Complicated Relationship: Glycosylation, Ca(II), and Primary Sequence Affect the Interactions and Kinetics between Two Model Mollusk Shell Intracrystalline Nacre Proteins

The formation of the mollusk shell requires the participation of proteins, many of which may be interactive with one another. We examined a model protein pair system from the mollusk Haliotis rufescens, wherein we probed the interactions between recombinant forms of two major nacre layer proteins, AP7, and the glycoprotein, AP24. Here, the focus was on the impact that the AP24 glycosylation and primary sequence had on AP24-AP7 binding. We find that both the glycosylated and nonglycosylated variants of AP24 bound to AP7 but with different quantities, kinetics, and internal rearrangements. Moreover, the binding of AP7 with nonglycosylated and glycosylated AP24 was found to be Ca(II)-dependent and -independent, respectively. Yet both variants of AP24 combine with AP7 to form hybrid hydrogel particles that are similar in their physical properties. Thus, AP7 and AP24 protein sequences are interactive and form hydrogels, but the interactions are tuned by glycosylation and Ca(II). These features may have an impact on the nacre matrix formation.

Identification of nacre matrix protein genes hic14 and hic19 and their roles in crystal growth and pearl formation in the mussel Hyriopsis cumingii

Biological mineralization is a highly programmed process in which inorganic minerals reassociate under the strict control of the extracellular matrix to form minerals with special functions and patterns. Shells are biominerals, and their synthesis is finely regulated by organic matrix including matrix proteins, polysaccharides, lipids, pigments, free amino acids, and small peptides. In this study, two matrix protein genes, hic14 and hic19, were isolated from the mantle of the mussel Hyriopsis cumingii. Tissue expression analysis showed that both proteins were expressed mainly in the mantle, and in situ hybridization of mantle tissues showed that they were specifically expressed in the dorsal epithelial cells of mantle pallial. Therefore, hic14 and hic19 were both nacreous layer matrix proteins. In the pearl insertion experiment, hic14 and hic19 kept low expression during pearl sac formation and disordered calcium carbonate deposition, and increased significantly during pearl nacre accumulation, which showed that both proteins participated in the mineralization of pearl nacre. In the RNA interference experiment, shell nacre tablet growth was inhibited after crystal nucleation due to the decreased expression of hic14, and crystal morphology and arrangement of nacre were highly modified after expression of hic19 was inhibited. These results provided further evidence that both hic14 and hic19 participated in nacreous layer biomineralization.

Composite Materials Design: Biomineralization Proteins and the Guided Assembly and Organization of Biomineral Nanoparticles

There has been much discussion of the role of proteins in the calcium carbonate biomineralization process, particularly with regard to nucleation, amorphous stabilization/transformation, and polymorph selection. However, there has been little if any discussion of the potential role that proteins might play in another important process: the guided assembly and organization of mineral nanoparticles into higher-ordered structures such as mesocrystals. This review discusses particle attachment theory and recent evidence of mineral-associated proteins forming hydrogels that assemble and organize mineral clusters into crystalline phase. From this discussion we postulate a mechanism by which biomineralization protein hydrogel aggregation assists in mineral nanoparticle assembly and organization within calcium carbonate skeletal elements and discuss potentials ways for harnessing this process in materials design.

Intracrystalline incorporation of nacre protein hydrogels modifies the mechanical properties of calcite crystals: a microcompression study

The fracture toughness of mollusk shell nacre has been attributed to many factors, one of which is the intracrystalline incorporation of nacre-specific proteins.