Sodium-dependent phosphate transporter protein 1 is involved in the active uptake of inorganic phosphate in nephrocytes of the kidney and the translocation of Pi into the tubular epithelial cells in the outer mantle of the giant clam, Tridacna squamosa

Abstract

Giant clams display light-enhanced inorganic phosphate (P_i) absorption, but how the absorbed (P_i) is translocated to the symbiotic dinoflagellates living extracellularly in a tubular system is unknown. They can accumulate (P_i) in the kidney, but the transport mechanism remains enigmatic. This study aimed to elucidate the possible functions of sodium-dependent phosphate transporter protein 1-homolog (PiT1-like), which co-transport Na⁺ and H₂PO₄^–, in these two processes. The complete cDNA coding sequence of PiT1-like, which comprised 1,665 bp and encoded 553 amino acids (59.3 kDa), was obtained from the fluted giant clam, Tridacna squamosa. In the kidney, PiT1-like was localized in the plasma membrane of nephrocytes, and could therefore absorb (P_i) from the hemolymph. As the gene and protein expression levels of PiT1-like were up-regulated in the kidney during illumination, PiT1-like could probably increase the removal of (P_i) from the hemolymph during light-enhanced (P_i) uptake. In the ctenidial epithelial cells, PiT1-like had a basolateral localization and its expression was also light-dependent. It might function in (P_i) sensing and the absorption of (P_i) from the hemolymph when (P_i) was limiting. In the outer mantle, PiT1-like was localized in the basolateral membrane of epithelial cells forming the tertiary tubules. It displayed light-enhanced expression levels, indicating that the host could increase the translocation of Pi from the hemolymph into the tubular epithelial cells and subsequently into the luminal fluid to support increased (P_i) metabolism in the photosynthesizing dinoflagellates. Taken together, the accumulation of (P_i) in the kidney of giant clams might be unrelated to limiting the availability of (P_i) to the symbionts to regulate their population.