Inorganic Overgrowth of Aragonite on Molluscan Nacre Examined by Atomic Force Microscopy

A Novel Kunitz-Type Serine Protease Inhibitor (HcKuSPI) is Involved in Antibacterial Defense in Innate Immunity and Participates in Shell Formation of Hyriopsis cumingii

Serine protease inhibitors (SPIs) are abundantly reported for its inhibition against specific proteases involved in the immune responses, but SPI data related to calcareous shells are scarce. Previously, our research group has reported the proteome analysis of non-nucleated pearl powder, and a candidate matrix protein containing two Kunitz domains in the acid soluble fraction caught our attention. In the present study, the full-length cDNA sequence of HcKuSPI was obtained from Hyriopsis cumingii. HcKuSPI was specifically expressed in the mantle, with hybridization signals mainly concentrated to dorsal epithelial cells at the mantle edge and weak signals at the mantle pallium, suggesting HcKuSPI was involved in shell formation. HcKuSPI expression in the mantle was upregulated after Aeromonas hydrophila and Staphylococcus aureus challenge to extrapallial fluids (EPFs). A glutathione S transferase (GST)-HcKuSPI recombinant protein showed strong inhibitory activity against the proteases, trypsin and chymotrypsin. Moreover, HcKuSPI expression in an experimental group was significantly higher when compared with a control group during pellicle growth and crystal deposition in shell regeneration processes, while the organic shell framework of newborn prisms and nacre tablets was completely destroyed after HcKuSPI RNA interference (RNAi). Therefore, HcKuSPI secreted by the mantle may effectively neutralize excess proteases and bacterial proteases in the EPF during bacterial infection and could prevent matrix protein extracellular degradation by suppressing protease proteolytic activity, thereby ensuring a smooth shell biomineralization. In addition, GST-HcKuSPI was also crucial for crystal morphology regulation. These results have important implications for our understanding of the potential roles of SPIs during shell biomineralization.

Predicting hydration layers on surfaces using deep learning

Characterisation of the nanoscale interface formed between minerals and water is essential to the understanding of natural processes, such as biomineralization, and to develop new technologies where function is dominated by the mineral-water interface. Atomic force microscopy offers the potential to characterize solid-liquid interfaces in high-resolution, with several experimental and theoretical studies offering molecular scale resolution by linking measurements directly to water density on the surface. However, the theoretical techniques used to interpret such results are computationally intensive and development of the approach has been limited by interpretation challenges. In this work, we develop a deep learning architecture to learn the solid-liquid interface of polymorphs of calcium carbonate, allowing for the rapid predictions of density profiles with reasonable accuracy.

A Kunitz proteinase inhibitor (HcKuPI) participated in antimicrobial process during pearl sac formation and induced the overgrowth of calcium carbonate in Hyriopsis cumingii

Proteinase inhibitors with the ability to inhibit specific proteinases are usually closely connected with the immune system. Interestingly, proteinase inhibitors are also a common ingredient in the organic matrix of mollusk shells. However, the molecular mechanism that underlies the role of proteinase inhibitors in immune system and shell mineralization is poorly known. In this study, a Kunitz serine proteinase inhibitor (HcKuPI) was isolated from the mussel Hyriopsis cumingii. HcKuPI was specifically expressed in dorsal epithelial cells of the mantle pallium and HcKuPI dsRNA injection caused an irregular surface and disordered deposition on the aragonite tablets of the nacreous layer. These results indicated that HcKuPI plays a vital role in shell nacreous layer biomineralization. Moreover, the expression pattern of HcKuPI during LPS challenge and pearl formation indicated its involvement in the antimicrobial process during pearl sac formation and nacre tablets accumulation during pearl formation. In the in vitro calcium carbonate crystallization assay, the addition of GST-HcKuPI increased the precipitation rate of calcium carbonate and induced the crystal overgrowth of calcium carbonate. Taken together, these results indicate that HcKuPI is involved in antimicrobial process during pearl formation, and participates in calcium carbonate deposition acceleration and morphological regulation of the crystals during nacreous layer formation. These findings extend our knowledge of the role of proteinase inhibitors in immune system and shell biomineralization.

Molecular cloning and characterization of Pif gene from pearl mussel, Hyriopsis cumingii, and the gene expression analysis during pearl formation

In the present study, the Pif gene of the freshwater pearl aquaculture mussel, Hyriopsis cumingii (HcPif) was successfully cloned and functionally characterized. The full sequence of HcPif gene consists of 3415 base pairs, which putatively encode two proteins, HcPif90 and HcPif80. A sequence analysis revealed that HcPif contained a von Willebrand factor type A domain and a chitin-binding domain, and shared many functional residues with other Pif homologues. A highly conserved sequence, FKGLDEIELML, at the C-terminus of Pif80s was identified as the key functional site. The corresponding peptide fragment markedly modified the morphology of calcite crystallites in CaCO₃ crystallization assay and might play an essential role in the interactive binding between HcPif80 and CaCO₃. Moreover, real-time PCR results showed that HcPif gene was dominantly expressed in the pearl secreting tissues and its expression changed in response to the different development status of the pearl sac during pearl aquaculture. The gene expression of HcPif was maximum 7 days after mantle grafting and declined to about the control level on day 30. Our in vitro and in vivo experimental data indicated that HcPif gene possessed the inherent characteristics of a nacre formation gene and its expression might faithfully reflect the pearl secretion status of the pearl mussels examined. Our findings may extend the understanding of the biomineralization mechanism of nacre formation and provide a potential biomarker for pearl farming.

Molecular approaches to understand biomineralization of shell nacreous layer

The nacreous layer of molluskan shells, which consists of highly oriented aragonitic crystals and an organic matrix (including chitin and proteins), is a product of biomineralization. This paper briefly introduces the recent research advances on nacre biomineralization of shells from bivalves and gastropods, which mainly focus on analysis of the micro- and nano-structure and components of shell nacreous layers, and investigations of the characteristics and functions of matrix proteins from nacre. Matrix proteins not only participate in construction of the organic nacre framework, but also control the nucleation and growth of aragonitic crystals, as well as determine the polymorph specificity of calcium carbonate in nacre. Moreover, the inorganic aragonite phase also plays an active role in organizing nacre microstructure. Based on these studies, several models to illustrate the formation mechanism related to lamellar nacre in bivalves, and columnar nacre in gastropods are introduced.

Morphological control of CaCO3 films with large area: effect of additives and self-organization under atmospheric conditions

Hierarchically structured CaCO(3) films were synthesized at atmospheric conditions (room temperature and 1 atm) without the use of templates or amphiphilic molecules in this process. The resulting CaCO(3) film was formed by self-organization between Ca(OH)(2) and aqueous CO(2). The building blocks of the CaCO(3) film were thought to be CaCO(3) primary nanoparticles that aligned to build higher level structures with greater size, called mesocrystals, depending on the additives. The soluble additives played a key role in the control of the morphology, crystallinity, and polymorphism of the CaCO(3) film, and the effects strongly depended on the type of additive and their concentrations. The additives used in this study decreased the crystallinity of CaCO(3) (calcite) film in the order of glucose > aspartic acid > serine in a manner inversely proportional to the concentration of the additives. In addition, Mg(2+), K(+), and Na(+) ion additives led to the formation of an aragonite phase, the proportion of which increased with the concentration of ions. The threshold concentrations of these ions for the formation of the aragonite phase in CaCO(3) film were found to be in the order of Na(+) > K(+) > Mg(2+).

Recombinant perlucin nucleates the growth of calcium carbonate crystals: molecular cloning and characterization of perlucin from disk abalone, Haliotis discus discus

Perlucin is well known as an important functional protein regulating pearl formation and shell biomineralization. In this study, we cloned the perlucin gene from the abalone Haliotis discus discus cDNA library. The full-length cDNA of the abalone H. discus discus perlucin gene consisted of 1038 bp nucleotides, encoding a putative signal peptide of 22 amino acids and a mature protein of 129 amino acids, which shared 55% identity with the homologous protein in greenlip abalone. The mature protein coding sequence was inserted into pMal-c2X expression vector and it expressed the recombinant protein in E. coli (Rosetta-gammi DE3). The maltose binding protein (MBP) fusion perlucin successfully promoted calcium carbonate precipitation and directed calcite crystal morphological modification. The successful expression of active recombinant perlucin suggested that recombinant perlucin gene transfer has the capability by color modification to improve the pearl's value. In the view of molecular structure, perlucin was a typical C-type lectin, which contained one highly conserved carbohydrate recognition domain. Reverse transcription polymerase chain reaction (RT-PCR) results showed that perlucin gene was expressed not only in the mantle, but also in the gill and digestive tract. Further characterization of perlucin in abalone non-self recognition and disease resistance is promising and anticipated.

Otolith crystals (in Carapidae): Growth and habit

The biomineralization of otoliths results mainly from the release of soluble Ca(2+), which is in turn precipitated as CaCO(3) crystals. In some Carapidae, sagittae sections have been shown to reveal a three-dimensional asymmetry with a nucleus close to the sulcal side, an unusual position. This study seeks to understand otolith formation in Carapus boraborensis. The unusual shape of the otolith is partly explained by the distribution of the epithelium cells, and particularly the sensory epithelium. Experimental evidence shows for the first time that aragonite growth takes place along the c-axis. These aragonite needles present two different habits. On the sulcal side is found the acicular form resulting from rapid growth during a short period of time. On the anti-sulcal side, the prismatic form seen there is due to a slower growth speed over longer periods. The otolith surface was observed each hour during a period of 24h in fishes reared in similar conditions. This allowed for the first time the direct observation on the otolith surface of the deposition of the two layers (L-zone and D-zone). In C. boraborensis, the organic-rich layer (D-zone) develops during the day, whereas the CaCO(3) layer (L-zone) seems to be deposited during the night.

A novel extrapallial fluid protein controls the morphology of nacre lamellae in the pearl oyster, Pinctada fucata

Mollusk shell nacre is known for its superior mechanical properties and precisely controlled biomineralization process. However, the question of how mollusks control the morphology of nacre lamellae remains unresolved. Here, a novel 38-kDa extrapallial fluid (EPF) protein, named amorphous calcium carbonate-binding protein (ACCBP), may partially answer this question. Although sequence analysis indicated ACCBP is a member of the acetylcholine-binding protein family, it is actively involved in the shell mineralization process. In vitro, ACCBP can inhibit the growth of calcite and induce the formation of amorphous calcium carbonate. When ACCBP functions were restrained in vivo, the nacre lamellae grew in a screw-dislocation pattern, and low crystallinity CaCO(3) precipitated from the EPF. Crystal binding experiments further revealed that ACCBP could recognize different CaCO(3) crystal phases and crystal faces. With this capacity, ACCBP could modify the morphology of nacre lamellae by inhibiting the growth of undesired aragonite crystal faces and meanwhile maintain the stability of CaCO(3)-supersaturated body fluid by ceasing the nucleation and growth of calcite. Furthermore, the crystal growth inhibition capacity of ACCBP was proved to be directly related to its acetylcholine-binding site. Our results suggest that a "safeguard mechanism" of undesired crystal growth is necessary for shell microstructure formation.

Microstructure of Monoplacophora (Mollusca) shell examined by low-voltage field emission scanning electron and atomic force microscopy

The shell of Micropilina arntzi (Mollusca: Monoplacophora), a primitive molluscan class, was examined by using field emission scanning electron microscopy (FESEM) at low voltage and atomic force microscopy (AFM). The use of these two techniques allowed the observation of fine details of Micropilina arntzi shell and contributed to bring new features concerning the study of molluscan shell microtexture. Imaging with low-voltage FESEM provided well-defined edge contours of shell structures, while analyzing the sample with AFM gave information about the step height of stacked internal structures as well as the dimension of the particles present in their surface at a nanometric level. The shell microstructure of Monoplacophora species presents different patterns and may be a taxonomic implication in the systematic studies of the group.

Mineral characterization in calcifying tissues: atomic, molecular and macromolecular perspectives

This paper presents a general overview of calcification and involves aspects of the chemical, physical, and biological nature of mineral crystals in invertebrate and vertebrate tissues, with selected examples of the latter. Two broad areas are described: mineral structure and composition. Mineral formation is detailed in an incidental fashion. Both classical research and recent data appropriate to mineralization studies are noted in order to convey basic principles, as well as the sense and direction of current investigations on the mineral phases of calcified tissues. In this context, novel analytical and imaging techniques from a number of different laboratories lately have helped characterize crystal size, shape, and composition; mineral association with respect to collagen; atomic lattice structure of crystal surfaces; interrelationships between non-collagenous matrix components and mineral; and stereochemical organization of putative matrix nucleation sites. Together, this work has provided a more complete understanding of the mineral-matrix atomic, molecular, and macromolecular interactions that underlie the general mechanism of calcification in biological tissues.