cDNA cloning and functional expression of a Ca2+-sensing receptor with truncated C-terminal tail from the mozambique tilapia (Oreochromis mossambicus)

Abstract

The complete cDNA sequence of the tilapia extracellular Ca 2+-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 Ser169-171) 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+Io 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 phospliolipase C activation to [Ca2+] o was dependent on the ionic strength of the bathing medium, supporting a role in salinity sensing.