Transepithelial absorption of exogenous inorganic carbon in the ctenidium of the giant clam, Tridacna squamosa involves a basolateral electrogenic Na+-HCO3- cotransporter 1 that displays light-enhanced gene and protein expression levels

Abstract

Giant clams live in symbiosis with phototrophic dinoflagellates. They need to increase the uptake of inorganic carbon (Ci) from the ambient seawater to support light-enhanced shell formation in the host and photosynthesis in the symbionts during illumination. The ctenidium is the major site of light-enhanced Ci absorption in the fluted giant clam, Tridacna squamosa. Catalyzed by dual-domain carbonic anhydrase, exogenous HCO₃⁻ is dehydrated to CO₂, which permeates the apical membrane of the ctenidial epithelium and is rehydrated back to HCO₃⁻ in the cytoplasm. However, the molecular mechanism that transports cytoplasmic HCO₃⁻ through the basolateral membrane to the hemolymph has not been elucidated. We have obtained from the ctenidium of T. squamosa the complete cDNA coding sequence of a homolog of electrogenic Na⁺–HCO₃⁻ cotransporter 1 (NBCe1-like), which comprised 3450 bp, encoding a protein (NBCe1-like) of 1142 amino acids and 128.9 kDa. NBCe1-like had a basolateral localization in epithelial cells covering the ctenidial filament and those surrounding the tertiary water channels. Light exposure led to significant increases in the transcript and protein levels of NBCe1-like/NBCe1-like in the ctenidium of T. squamosa, indicating that NBCe1-like could be involved in the increased transport of cytoplasmic HCO₃⁻ across the basolateral membrane into the hemolymph during illumination. Additionally, NBCe1-like might also participate in light-enhanced NO3− absorption in T. squamosa, due to the replacement of aspartate (residue 585) with glutamine. Exogenous NO3− could be absorbed by the ctenidial epithelial cells through the apical H+:NO3− cotransporter (SIALIN) and then transported through the basolateral NBCe1-like to the hemolymph.