Natural Sciences and Science Education, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616, Republic of Singapore.
Department of Biological Sciences, National University of Singapore, Kent Ridge, Singapore 117543, Republic of Singapore.
Comp Biochem Physiol A Mol Integr Physiol. 2020 Dec;250:110791. doi: 10.1016/j.cbpa.2020.110791. Epub 2020 Aug 13.
The colorful outer mantle of giant clams contains abundance of symbiotic dinoflagellates (zooxanthellae) and iridocytes, and has direct exposure to light. In light, photosynthesizing dinoflagellates produce O, and the host cells in the outer mantle would be confronted with hyperoxia-related oxidative stress. In comparison, the whitish inner mantle contains few symbiotic dinoflagellates and no iridocytes. It is involved in shell formation, and is shaded from light. CuZnSOD is a cytosolic enzyme that scavenges intracellular O. We had obtained from the outer mantle of the fluted giant clam, Tridacna squamosa, the complete cDNA coding sequence of a host-derived copper zinc superoxide dismutase (CuZnSOD), which comprised 462 bp and encoded for 154 amino acids with a calculated MW of 15.6 kDa. CuZnSOD was expressed strongly in the outer mantle, ctenidium, hepatopancreas and kidney. The transcript level of CuZnSOD remained unchanged in the outer mantle during light exposure, but the protein abundance of CuZnSOD increased ~3-fold after exposure to light for 6 or 12 h. By contrast, 12 h of light exposure had no significant effects on the gene and protein expression levels of CuZnSOD/CuZnSOD in the inner mantle. Hence, the increased expression of CuZnSOD in the outer mantle of T. squamosa was probably a host's response to ameliorate oxidative stress related to photosynthesis in the symbionts, and not simply due to increased metabolic rate in the host cells. Evidently, the host clam must possess light- or O-responsive anti-oxidative defenses in order to align with the light-dependent photosynthetic activity of its symbionts.
巨蛤的彩色外壳含有丰富的共生甲藻（虫黄藻）和虹彩细胞，并且直接暴露在光线下。在光线下，进行光合作用的甲藻会产生 O，而外壳中的宿主细胞将面临与高氧相关的氧化应激。相比之下，白色的内壳含有较少的共生甲藻和没有虹彩细胞。它参与贝壳的形成，并避免受到光照。CuZnSOD 是一种细胞溶质酶，可清除细胞内的 O。我们从波纹巨蛤（Tridacna squamosa）的外壳中获得了宿主来源的铜锌超氧化物歧化酶（CuZnSOD）的完整 cDNA 编码序列，该序列包含 462bp，编码 154 个氨基酸，计算分子量为 15.6 kDa。CuZnSOD 在外壳、鳃、肝胰腺和肾脏中强烈表达。在暴露于光线下时，CuZnSOD 的转录水平在外壳中保持不变，但暴露于光线下 6 或 12 小时后，CuZnSOD 的蛋白丰度增加了约 3 倍。相比之下，12 小时的光照对虹彩细胞中 CuZnSOD/CuZnSOD 的基因和蛋白表达水平没有显著影响。因此，T. squamosa 外壳中 CuZnSOD 的表达增加可能是宿主为缓解共生体光合作用相关的氧化应激而做出的反应，而不仅仅是由于宿主细胞代谢率的增加。显然，宿主蛤必须具有对光或 O 有响应的抗氧化防御机制，以与共生体的光依赖性光合作用活动相协调。
