Salinity stress induces paradoxical cyclin D2 expression via transcription-translation decoupling in Hong Kong oysters (Crassostrea hongkongensis)

The Hong Kong oyster ( Crassostrea hongkongensis ), an economically important shellfish species in the South China Sea, faces significant challenges in salinity adaptation due to the expansion of off-bottom farming. Previous studies have shown that salinity fluctuations can regulate the osmoregulatory capacity of bivalves by affecting cell cycle progression, yet the roles of cyclin D2 as a key cell cycle regulator remain unexplored in this process. In this study, we cloned the full-length cDNA sequence of C. hongkongensis cyclin D2 ( Ch-CCND2 ) from a gill cDNA library for the first time. The Ch-CCND2 cDNA was 3735 bp in length, and contained an ORF of 876 bp, a 5’-UTR of 229 bp and a 3’-UTR of 2630 bp. Ch-CCND2 protein sequence was 291 amino acids long, and shared high homology with cyclin D2 from other molluscan species. Ch-CCND2 mRNA was expressed in all examined tissues, with high expression levels found in the labial palp, gill and gonad. Mouse anti-Ch-CCND2 antiserums were generated using corresponding synthetic peptides as antigens, and the immunohistochemical localization revealed that Ch-CCND2 protein is abundantly expressed in the gill and specifically distributed in the surface cells on both sides near the bases of the ordinary and principal filaments of the gill. During a 10-day salinity exposure experiment, both hypo- (5 ppt) and hyper- (25 and 35 ppt) salinity treatments significantly reduced Ch-CCND2 mRNA levels in the gill. However, under hypo-salinity conditions, the protein expression of Ch-CCND2 in the gill was markedly upregulated. This study reports a transcriptional-translational decoupling phenomenon of cyclin D2 in marine invertebrates, suggesting its special gene expression regulation through post-translational mechanisms underlying salinity adaptation. Our results provide critical theoretical foundations for developing salinity-resistant strains of Hong Kong oysters and other economically important shellfish species. • The full-length CCND2 cDNA was first isolated from Hong Kong oyster using the 3 ´ /5 ´ RACE system. • Phylogenetic analysis segregated molluscan cyclin D2 from chordate cyclin D2 and other D-type cyclins. • The Ch-CCND2 mRNA is ubiquitous, with high levels in the labial palp, gill, and gonad. • Ch-CCND2 protein localizes to the basal surface cells of ordinary and principal gill filaments. • Ch-CCND2 protein abundance significantly increased under hypo-salinity conditions, decoupling from mRNA levels.