Exosome-Secreted Tropomyosin and Gigasin-6 Roles in Biomineralization Divergence Between Estuarine and Coastal...

Biomineralization in mollusks, a fundamental process in marine ecosystems, is highly sensitive to anthropogenic stressors. Exosome-secreted species-specific shell matrix proteins (SMPs) are essential in biomineralization adaptation but remain understudied. Estuaries are considered unfavorable for biomineralization compared to open coastal zones and serve as an ideal research location to explore the roles of exosome-secreted species-specific SMPs in biomineralization adaptation under future rapid environmental change. Here, combining proteomics of shell matrix and mantle-derived exosomes, the high-abundance species-specific SMPs Car-TPM (tropomyosin from estuarine oyster Crassostrea ariakensis) and Cgi-GIGA6 (gigasin-6 from coastal oyster Crassostrea gigas) were taken as representatives to decipher the roles of exosome-secreted species-specific SMPs in oyster biomineralization adaptation. Tissue expression profiles and in situ hybridization revealed that Car-TPM was highly expressed in the adductor muscle and mantle, while Cgi-GIGA6 predominated in the mantle. Post-injury experiments demonstrated that Car-TPM expression upregulated quickly at 6 h, and Cgi-GIGA6 continued to be down-regulated. Knockdown of Car-TPM suppressed shell repair, whereas silencing Cgi-GIGA6 enhanced it. In vitro assays revealed that Car-TPM significantly promoted calcium carbonate precipitation and aggregation of rhombohedral calcite crystals, whereas Cgi-GIGA6 suppressed crystallization and eroded the original flat edges. These findings indicate that Car-TPM is a positive regulator of biomineralization in C. ariakensis inhabiting harsh estuarine environments, while Cgi-GIGA6 exerts a negative regulatory effect to optimize energy allocation by restraining excessive biomineralization in C. gigas. This study reveals the essential role of species-specific SMPs secreted via exosomes in the biomineralization adaptation and adaptive potential of mollusks in future marine environments.