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Molecular Cloning of Otoconin-22 Complementary Deoxyribonucleic Acid in the Bullfrog Endolymphatic Sac: Effect of Calcitonin on Otoconin-22 Messenger Ribonucleic Acid Levels

Department of Biology (Y.Y., M.S., S.T.), Faculty of Science, Shizuoka University, Shizuoka 422-8529, Japan; Institute for Molecular and Cellular Regulation (H.T.), Gunma University, Maebashi 371-8512, Japan; Division of Biodiversity (Y.S.), Noto Marine Laboratory, Institute of Nature and Environmental Technology, Kanazawa University, Kanazawa 927-0553, Japan; and Department of Biology (S.K.), School of Education, Waseda University, Tokyo 169-8050, Japan

Address all correspondence and requests for reprints to: Dr. Shigeyasu Tanaka, Department of Biology, Faculty of Science, Shizuoka University, Ohya 836, Shizuoka 422-8529, Japan. E-mail: sbstana{at}ipc shizuoka.ac.jp.

Anuran amphibians have a special organ called the endolymphatic sac (ELS), containing many calcium carbonate crystals, which is believed to have a calcium storage function. The major protein of aragonitic otoconia, otoconin-22, which is considered to be involved in the formation of calcium carbonate crystals, has been purified from the saccule of the Xenopus inner ear. In this study, we cloned a cDNA encoding otoconin-22 from the cDNA library constructed for the paravertebral lime sac (PVLS) of the bullfrog, Rana catesbeiana, and sequenced it. The bullfrog otoconin-22 encoded a protein consisting of 147 amino acids, including a signal peptide of 20 amino acids. The protein had cysteine residues identical in a number and position to those conserved among the secretory phospholipase A₂ family. The mRNA of bullfrog otoconin-22 was expressed in the ELS, including the PVLS and inner ear. This study also revealed the presence of calcitonin receptor-like protein in the ELS, with the putative seven-transmembrane domains of the G protein-coupled receptors. The ultimobranchialectomy induced a prominent decrease in the otoconin-22 mRNA levels of the bullfrog PVLS. Supplementation of the ultimobranchialectomized bullfrogs with synthetic salmon calcitonin elicited a significant increase in the mRNA levels of the sac. These findings suggest that calcitonin secreted from the ultimobranchial gland, regulates expression of bullfrog otoconin-22 mRNA via calcitonin receptor-like protein on the ELS, thereby stimulating the formation of calcium carbonate crystals in the lumen of the ELS.

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				Y. Yaoi, T. Onda, Y. Hidaka, S. Yajima, M. Suzuki, and S. Tanaka Developmental Expression of Otoconin-22 in the Bullfrog Endolymphatic Sac and Inner Ear J. Histochem. Cytochem., May 1, 2004; 52(5): 663 - 670. [Abstract] [Full Text] [PDF]

				Y. Yaoi, M. Suzuki, H. Tomura, S. Kurabuchi, Y. Sasayama, and S. Tanaka Expression and Localization of Prohormone Convertase PC1 in the Calcitonin-producing Cells of the Bullfrog Ultimobranchial Gland J. Histochem. Cytochem., November 1, 2003; 51(11): 1459 - 1466. [Abstract] [Full Text] [PDF]

				E. Schipani Otoconin-22 and Calcitonin: A Novel Modality of Regulating Calcium Storages in Lower Vertebrates? Endocrinology, August 1, 2003; 144(8): 3285 - 3286. [Full Text] [PDF]