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Illumination enhances the protein abundance of sarcoplasmic reticulum Ca2+-ATPases-like transporter in the ctenidium and whitish inner mantle of the giant clam, Tridacna squamosa, to augment exogenous Ca2+ uptake and shell formation, respectively

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Date
2021
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Abstract
The fluted giant clam, Tridacna squamosa, can perform light-enhanced shell formation, aided by its symbiotic dinoflagellates <i>(Symbiodinium, Cladocopium, Durusdinium)</i>, which are able to donate organic nutrients to the host. During light-enhanced shell formation, increased Ca<sup>2+</sup>transport from the hemolymph through the shell-facing epithelium of the inner mantle to the extrapallial fluid, where calcification occurs, is necessary. Additionally, there must be increased absorption of exogenous Ca<sup>2+</sup>from the surrounding seawater, across the epithelial cells of the ctenidium (gill) into the hemolymph, to supply sufficient Ca<sup>2+</sup>for light-enhanced shell formation. When Ca<sup>2+</sup>moves across these epithelial cells, the low intracellular Ca<sup>2+</sup>concentration must be maintained. <i>Sarco</i> (endo)plasmic reticulum Ca<sup>2+</sup>-ATPase (SERCA) regulates the intracellular Ca<sup>2+</sup>concentration by pumping Ca<sup>2+</sup>into the sarcoplasmic/endoplasmic reticulum (SR/ER) and Golgi apparatus. Indeed, the ctenidium and inner mantle of <i>T. squamosa</i>, expressed a homolog of <i>SERCA (SERCA</i>-like transporter) that consists of 3009 bp, encoding 1002 amino acids of 110.6 kDa. SERCA-like-immunolabeling was non-uniform in the cytoplasm of epithelial cells of ctenidial filaments, and that of the shell-facing epithelial cells of the inner mantle. Importantly, the protein abundance of SERCA-like increased significantly in the ctenidium and the inner mantle of <i>T. squamosa</i> after 12 h and 6 h, respectively, of light exposure. This would increase the capacity of pumping Ca<sup>2+</sup>into the endoplasmic reticulum and avert a possible surge in the cytosolic Ca<sup>2+</sup>concentration in epithelial cells of the ctenidial filaments during light-enhanced Ca<sup>2+</sup>absorption, and in cells of the shell-facing epithelium of the inner mantle during light-enhanced shell formation.
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This is the original draft, prior to peer-review, of a manuscript published in Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology. The published version is available online at https://doi.org/10.1016/j.cbpa.2020.110811
Keywords
Calcification, Mantle, Symbiosis, Tridacnid, Zooxanthellae
Citation
Chan, J. W. J., Boo, M. V., Wong, W. P., Chew, S. F., & Ip, Y. K. (2021). Illumination enhances the protein abundance of sarcoplasmic reticulum Ca2+-ATPases-like transporter in the ctenidium and whitish inner mantle of the giant clam, Tridacna squamosa, to augment exogenous Ca2+ uptake and shell formation, respectively. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 251, Article 110811. https://doi.org/10.1016/j.cbpa.2020.110811
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