Shell repair and the potential microbial causal in a shell disease of the pearl oyster Pinctada fucata

The molecular response of Mytilus coruscus mantle to shell damage under acute acidified sea water revealed by iTRAQ based quantitative proteomic analysis

Mytilus coruscus is an economically important marine bivalve that lives in estuarine sea areas with seasonal coastal acidification and frequently suffers shell injury in the natural environment. However, the molecular responses and biochemical properties of Mytilus under these conditions are not fully understood. In the present study, we employed tandem mass spectrometry combined with isobaric tagging to identify differentially expressed proteins in the mantle tissue of M. coruscus under different short-term treatments, including shell-complete mussels raised in normal seawater (pH 8.1), shell-damaged mussels raised in normal seawater (pH 8.1), and acidified seawater (pH 7.4). A total of 2694 proteins were identified in the mantle, and analysis of their relative abundance from the three different treatments revealed alterations in the proteins involved in immune regulation, oxidation-reduction processes, protein folding and processing, energy provision, and cytoskeleton. The results obtained by quantitative proteomic analysis of the mantle allowed us to delineate the molecular strategies adopted by M. coruscus in the shell repair process in acidified environments, including an increase in proteins involved in oxidation-reduction processes, protein processing, and cell growth at the expense of proteins involved in immune capacity and energy metabolism. SIGNIFICANCE: The impact of global ocean acidification on calcifying organisms has become a major ecological and environmental problem in the world. Mytilus coruscus is an economically important marine bivalve living in estuary sea area with seasonal coastal acidification, and frequently suffering shell injury in natural environment. Molecular responses of M coruscus under the shell damage and acute acidification is still largely unknown. For this reason, iTRAQ based quantitative proteomic and histological analysis of the mantle from M. coruscus under shell damage and acute acidification were performed, for revealing the proteomic response and possible adaptation mechanism of Mytilus under combined shell damage and acidified sea water, and understanding how the mussel mantle implement a shell-repair process under acidified sea water. Our study provides important data for understanding the shell repair process and proteomic response of Mytilus under ocean acidification, and providing insights into potential adaptation of mussels to future global change.

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.

Metabolic profiling of Mytilus coruscus mantle in response of shell repairing under acute acidification

Mytilus coruscus is an economically important marine bivalve mollusk found in the Yangtze River estuary, which experiences dramatic pH fluctuations due to seasonal freshwater input and suffer from shell fracture or injury in the natural environment. In this study, we used intact-shell and damaged-shell M. coruscus and performed metabolomic analysis, free amino acids analysis, calcium-positive staining, and intracellular calcium level tests in the mantle to investigate whether the mantle-specific metabolites can be induced by acute sea-water acidification and understand how the mantle responds to acute acidification during the shell repair process. We observed that both shell damage and acute acidification induced alterations in phospholipids, amino acids, nucleotides, organic acids, benzenoids, and their analogs and derivatives. Glycylproline, spicamycin, and 2-aminoheptanoic acid (2-AHA) are explicitly induced by shell damage. Betaine, aspartate, and oxidized glutathione are specifically induced by acute acidification. Our results show different metabolic patterns in the mussel mantle in response to different stressors, which can help elucidate the shell repair process under ocean acidification. furthermore, metabolic processes related to energy supply, cell function, signal transduction, and amino acid synthesis are disturbed by shell damage and/or acute acidification, indicating that both shell damage and acute acidification increased energy consumption, and disturb phospholipid synthesis, osmotic regulation, and redox balance. Free amino acid analysis and enzymatic activity assays partially confirmed our findings, highlighting the adaptation of M. coruscus to dramatic pH fluctuations in the Yangtze River estuary.

Ca2+ addition facilitates the shell repair with eggs production of Pomacea canaliculata through biomineralization and food intaking regulation

Pomacea canaliculata was by far one of the most harmful invasive organisms in the world, causing serious harm to aquatic crops and ecosystem. Calcium carbonate is a common component of aquatic environment, which is important for the growth of Pomacea canaliculata. Therefore, the objective of this study was to investigate the response characteristics of P. canaliculata suffered shell breakage to the addition of calcium carbonate in water environment. In this experiment, we explored the effects of calcium carbonate addition on the P. canaliculata shell repair rate, food intake, egg production, shell strength, and calcium content through breaking the snails shell and the addition of calcium carbonate treatment. The results showed that snail broken-shell repaired mostly within 21 days. The snails experienced a significant increase in shell repair rates during earlier days of the treatment, especially for female snails. Food intake of snails exhibited different patterns when their shells were broken and calcium carbonate was added. Shell breakage treatment combined with calcium carbonate addition significantly increased the diameter of snail eggs compared with the control and the calcium carbonate addition treatment without shell-broken snail group. There was no significant difference in shell strength or calcium content of male snails between the treatments. The study suggests that P. canaliculata exhibits a sex-dependent response pattern when subjected to shell damage and calcium carbonate addition. The findings can provide some references to better understand the invasion mechanism and survival strategy of the P. canaliculata.

Direct control of shell regeneration by the mantle tissue in the pearl oyster Pinctada fucata

Molluscs rapidly repair the damaged shells to prevent further injury, which is vital for their survival after physical or biological aggression. However, it remains unclear how this process is precisely controlled. In this study, we applied scanning electronic microscopy and histochemical analysis to examine the detailed shell regeneration process in the pearl oyster Pinctada fucata. It was found that the shell damage caused the mantle tissue to retract, which resulted in relocation of the partitioned mantle zones with respect to their correspondingly secreting shell layers. As a result, the relocated mantle tissue dramatically altered the shell morphology by initiating de novo precipitation of prismatic layers on the former nacreous layers, leading to the formation of sandwich-like "prism-nacre-prism-nacre" structure. Real-time PCR revealed the up-regulation of the shell matrix protein genes, which was confirmed by the thermal gravimetric analysis of the newly formed shell. The increased matrix secretion might have led to the change of CaCO₃ precipitation dynamics which altered the mineral morphology and promoted shell formation. Taken together, our study revealed the close relationship between the physiological activities of the mantle tissue and the morphological change of the regenerated shells.

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.

Mass mortality of pearl oyster (Pinctada fucata (Gould)) in Japan in 2019 and 2020 is caused by an unidentified infectious agent

Mass mortality of 0-year-old pearl oysters, Pinctada fucata (Gould), and anomalies in adults were observed in Japan’s major pearl farming areas in the summer of 2019 and 2020. Although adult oyster mortality was low, both adult and juvenile oysters underwent atrophy of the soft body, detachment of the mantle from nacre (the shiny inner surface of the valves), deposition of brownish material on the nacre, and loss of nacre luster. Infection trials were conducted to verify the involvement of pathogens in this phenomenon. Healthy adult pearl oysters were obtained from areas where this disease had not occurred to use as the recipients. The sources of infection were either affected adult oysters with atrophied soft bodies or batches of juveniles in which mortality had reached conspicuous levels. Transmission of the disease to the healthy oysters were tested either by cohabitation with affected oysters or by injections of the hemolymph of affected animals. The injection infection test examined the effects of filtration and chloroform exposure on the pathogen. Occurrence of the disease was confirmed by the appearance of brown deposits on the nacre and loss of nacre luster. The abnormalities of nacre were clearly reproduced in recipient shells in three out of four cohabitation trials with affected oysters. The disease was also reproduced in six out of six injection trails either with hemolymph filtered through 100 nm filter or with hemolymph treated with chloroform. In a serial passage with hemolymph injections, the disease was successfully transmitted through eight passages. These results suggest that the etiology of the disease is a non-enveloped virus with a diameter ≤100 nm.