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J. Biol. Chem., Vol. 279, Issue 51, 53288-53297, December 17, 2004

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cDNA Cloning and Functional Expression of a Ca²+-sensing Receptor with Truncated C-terminal Tail from the Mozambique Tilapia (Oreochromis mossambicus)^*

Christopher A. Loretz, Catherine Pollina¶, Susumu Hyodo¶, Yoshio Takei¶, Wenhan Chang||, and Dolores Shoback||

From the National Science Foundation Tokyo Regional Office, American Embassy, 1-10-5, Akasaka, Minato-ku, Tokyo 107-8420, Japan, the ^¶Ocean Research Institute, The University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo 164-8639, Japan, and the ^||Endocrine Research Unit, Department of Veteran Affairs Medical Center, Department of Medicine, University of California, San Francisco, California 94121

The complete cDNA sequence of the tilapia extracellular Ca²⁺-sensing receptor (CaR) was determined. The transcript length of tilapia CaR (tCaR) is 3.4 kbp and encodes a 940-amino acid, 7-transmembrane domain protein that is consistent in its structural features with known mammalian and piscine CaRs. The tCaR extracellular domain includes a characteristic hydrophobic segment, conserved cysteine residues that are implicated in receptor dimerization (Cys¹²⁹ and Cys¹³¹) and in coupling to the transmembrane domain (nine conserved cysteine residues), and conserved serine residues (Ser¹⁴⁷ and Ser^169–171) that are linked to receptor binding of Ca²⁺ and L-amino acid-mediated potentiation of function. mRNA expression of tCaR was strong in kidney, brain, and gill. Weaker expression was observed in pituitary, stomach, intestine, urinary bladder, and heart. This distribution is consistent with possible physiological roles in endocrine cells, excitable tissues, and ion-transporting barrier epithelia. Expression of tCaR mRNA in kidney and intestine was salinity-dependent, suggesting a role for the receptor in iono-/osmoregulation in this euryhaline teleost species. Human embryonic kidney-293 cells transiently transfected with tCaR cDNA demonstrated dose-dependent phospholipase C activation in response to elevations in the extracellular Ca²⁺ concentration ([Ca²⁺]_o). Functional activation of the mitogen-activated protein kinase cascade by high [Ca²⁺]_o was also confirmed in these cells despite the naturally occurring truncation of the receptor's intracellular tail, which removes segments variably linked in mammalian CaRs to filamin-coupled activation of mitogen-activated protein kinase cascades. Sensitivity of phospholipase C activation to [Ca²⁺]_o was dependent on the ionic strength of the bathing medium, supporting a role in salinity sensing.

Received for publication, September 2, 2004

^* This work was supported by Japan Society for the Promotion of Science Grants-in-aid for Scientific Research (C) 14540609 (to S. H.) and (A) 13304063 (to Y. T.); by Japan Ministry of Education, Culture, Sports, Science and Technology Creative Basic Research Grant 12NP0201; and by U.S. Department of Veterans Affairs Merit Review and National Institutes of Health Grants R01 DK55846 (to D. S.) and R01 AG21353 (to W. C.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EBI Data Bank with accession number(s) AY541693.

To whom correspondence should be addressed. Tel.: 81-3-3224-5505; Fax: 81-3-3224-5507; E-mail: cloretz{at}nsf.gov.

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