Christabel Y. L. Chan, Kum C. Hiong, Celine Y. L. Choo, Mel V. Boo, Wai P. Wong, Shit F. Chew, and Yuen K. Ip

Giant clams contain phototrophic zooxanthellae, and live in nutrient-deficient tropical waters where light is available. We obtained the complete cDNA coding sequence of a homolog of mammalian sodium/glucose cotransporter 1 (SGLT1) – SGLT1-like – from the ctenidium of the fluted giant clam, Tridacna squamosa. SGLT1-like had a host origin and was expressed predominantly in the ctenidium. Molecular characterizations reveal that SGLT1-like of T. squamosa could transport urea, in addition to glucose, as other SGLT1s do. It has an apical localization in the epithelium of ctenidial filaments and water channels, and the apical anti-SGLT1-like immunofluorescence was stronger in individuals exposed to light than to darkness. Furthermore, the protein abundance of SGLT1-like increased significantly in the ctenidium of individuals exposed to light for 12 h, although the SGLT1-like transcript level remained unchanged. As expected, T. squamosa could perform light-enhanced glucose absorption, which was impeded by exogenous urea. These results denote the close relationships between light-enhanced glucose absorption and light-enhanced SGLT1-like expression in the ctenidium of T. squamosa. Although glucose absorption could be trivial compared with the donation of photosynthates from zooxanthellae in symbiotic adults, SGLT1-like might be essential for the survival of aposymbiotic larvae, leading to its retention in the symbiotic stage. A priori, glucose uptake through SGLT1-like might be augmented by the surface microbiome through nutrient cycling, and the absorbed glucose could partially fulfill the metabolic needs of the ctenidial cells. Additionally, SGLT1-like could partake in urea absorption, as T. squamosa is known to conduct light-enhanced urea uptake to benefit the nitrogen-deficient zooxanthellae.

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