Shell formation in the giant clam, Tridacna squamosa, may involve an apical Na+/Ca2+ exchanger 3 homolog in the shell-facing epithelium of the whitish inner mantle, which displays light-enhanced gene and protein expression

Abstract

Na+/Ca2+ exchangers (NCXs) transport one Ca2+ in exchange for three Na+ and may participate in the calcification processes of mammalian osteoblasts and invertebrate calcifying cells. The giant clam, Tridacna squamosa, conducts light-enhanced calcification with the aid of its symbiotic zooxanthellae. The objective of this study was to elucidate the role of NCX in shell formation in T. squamosa by cloning and characterizing a homolog of NCX (NCX3-like) from the whitish inner mantle and determining its subcellular localization, as well as examining the effects of light exposure on its transcript and protein expression levels therein. The cDNA coding sequence of NCX3-like from T. squamosa consisted of 2520 bp, encoding 539 amino acids of 92.5 kDa. NCX3-like immunolabeling was detected mainly in the apical epithelium of the shell-facing inner mantle and was apparently stronger in clams exposed to 12 h of light than the control kept in darkness for 12 h. Importantly, the transcript level and protein abundance of NCX3-like/NCX3-like increased significantly in the inner mantle after 6 h and 12 h of light exposure, respectively, as compared with the control. Taken together, these results indicate that NCX3-like might participate in light-enhanced calcification in T. squamosa. As the expression levels of plasma membrane Ca2+-ATPase (PMCA/PMCA) of the inner mantle are also known to increase during insolation, it is probable that expression levels of both PMCA/PMCA and NCX3-like/NCX3-like need to be up-regulated concomitantly in order to achieve saturating concentration of Ca2+ in the extrapallial fluid to augment shell formation during insolation.