The mantle exosome proteins of Hyriopsis cumingii participate in shell and nacre color formation

HcN57, A Novel Unusual Acidic Silk-Like Matrix Protein from Hyriopsis cumingii, Participates in Framework Construction and Nacre Nucleation During Nacreous Layer Formation

In the classic molecular model of nacreous layer formation, unusual acidic matrix proteins rich in aspartic acid (Asp) residues are essential for nacre nucleation due to their great affinity for binding calcium. However, the acidic matrix proteins discovered in the nacreous layer so far have been weakly acidic with a high proportion of glutamate. In the present study, several silk-like matrix proteins, including the novel matrix protein HcN57, were identified in the ethylenediaminetetraacetic acid-soluble extracts of the nacreous layer of Hyriopsis cumingii. HcN57 is a highly repetitive protein that consists of a high proportion of alanine (Ala, 34.4%), glycine (Gly, 22.5%), and serine (Ser, 11.4%). It forms poly Ala blocks, GlynX repeats, an Ala-Gly repeat, and a Ser-Ala-rich region, exhibiting significant similarity to silk proteins found in spider species. The expression of HcN57 was specifically located in the dorsal epithelial cells of the mantle pallium and mantle center. Notably, expression of HcN57 was relatively high during nacreous layer regeneration and pearl nacre deposition, suggesting HcN57 is a silk matrix protein in the nacreous layer. Importantly, HcN57 also contains a certain content of Asp residues, making it an unusual acidic matrix protein present in the nacreous layer. These Asp residues are mainly distributed in three large hydrophilic acidic regions, which showed inhibitory activity against aragonite deposition and morphological regulation of calcite in vitro. Moreover, HcN57-dsRNA injection resulted in failure of nacre nucleation in vivo. Taken together, our results show that HcN57 is a bifunctional silk protein with poly Ala blocks and Gly-rich regions that serve as space fillers within the chitinous framework to prevent crystallization at unnecessary nucleation sites and Asp-rich regions that create a calcium ion supersaturated microenvironment for nucleation in the center of nacre tablets. These observations contribute to a better understanding of the mechanism by which silk proteins regulate framework construction and nacre nucleation during nacreous layer formation.

Transcriptome and exosome proteome analyses provide insights into the mantle exosome involved in nacre color formation of pearl oyster Pinctada fucata martensii

Color polymorphisms in molluscan shells play an important economic in the aquaculture industry. Among bivalves, shell color diversity can reflect properties such as growth rate and tolerance. In pearl oysters, the nacre color of the donor is closely related to the pearl color. Numerous genes and proteins involved in nacre color formation have been identified within the exosomes of the mantle. In this study, we analyzed the carotenoids present in the mantle of gold- and silver-lipped pearl oysters, identifying capsanthin and xanthophyll as crucial pigments contributing to coloration. Transcriptome analysis of the mantle revealed several differentially expressed genes (DEGs) involved in color formation, including ferric-chelate reductase, mantle genes, and larval shell matrix proteins. We also isolated and identified exosomes from the mantles of both gold- and silver-lipped strains of the pearl oyster Pinctada fucata martensii, revealing the extracellular transition mechanism of coloration-related proteins. From these exosomes, we obtained a total of 1223 proteins, with 126 differentially expressed proteins (DEPs) identified. These proteins include those associated with carotenoid metabolism and Fe(III) metabolism, such as apolipoproteins, scavenger receptor proteins, β,β-carotene-15,15'-dioxygenase, ferritin, and ferritin heavy chains. This study may provide a new perspective on the nacre color formation process and the pathways involved in deposition within the pearl oyster P. f. martensii.

Circulating exosome miRNA, is it the novel nutrient molecule through cross-kingdom regulation mediated by food chain transmission from microalgae to bivalve?

MicroRNAs (miRNAs) can efficiently regulate gene expression at intracellular and extracellular levels. Plant-derived miRNAs are highly enriched in animal haemolymph and regulate mammalian gene expression. However, evidence for food-derived miRNAs in Mollusca species is lacking. In this study, we fed the microalga Nannochloropsis oculata to the pearl oyster Pinctada fucata martensii and detected dietary miRNAs in exosomes isolated from the haemolymph by RNA-seq. In total, 273 endogenous miRNAs were identified in all biological replicates. We identified 23 microalgae-derived miRNAs in the exosomes of pearl oyster haemolymph. Most microalgae-derived miRNAs showed high expression levels in both exosomes and microalgae and exhibited apparent variation among individuals. These food-derived miRNAs were predicted to participate in endocytosis, apoptosis, signal transduction, energy metabolism, and biomineralization by targeting multiple genes. These findings demonstrated the cross-kingdom transport of miRNAs from microalgae to bivalves and provide insights into novel nutrient transmission through the food chain.

Exosome-derived small non-coding RNAs reveal immune response upon grafting transplantation in Pinctada fucata (Mollusca)

Exosomes, a subset of small extracellular vesicles, carry various nucleic acids, proteins, lipids, amino acids and metabolites. They function as a mode of intercellular communication and molecular transfer. Exosome cargo molecules, including small non-coding RNAs (sncRNAs), are involved in the immune response in various organisms. However, the role of exosome-derived sncRNAs in immune responses in molluscs remains unclear. Here, we aimed to reveal the sncRNAs involved in the immune response during grafting transplantation by the pearl oyster Pinctada fucata. Exosomes were successfully extracted from the P. fucata haemolymph during graft transplantation. Abundant microRNAs (miRNAs) and PIWI-interacting RNAs (piRNAs) were simultaneously discovered in P. fucata exosomes by small RNA sequencing. The expression patterns of the miRNAs and piRNAs at the grafting and initial stages were not substantially different, but varied significantly between the initial and later stages. Target prediction and functional analysis indicate that these miRNAs and piRNAs are related to immune response upon grafting transplantation, whereas piRNAs may also be associated with transposon silencing by targeting with genome transposon elements. This work provides the basis for a functional understanding of exosome-derived sncRNAs and helps to gain further insight into the PIWI/piRNA pathway function outside of germline cells in molluscs.