Molecular cloning and characterization of a channel-like transporter mediating intestinal calcium absorption

Calcium is a major component of the mineral phase of bone and serves as a key intracellular second messenger. Postnatally, all bodily calcium must be absorbed from the diet through the intestine. Here we report the properties of a calcium transport protein (CaT1) cloned from rat duodenum using an expression cloning strategy in Xenopus laevis oocytes, which likely plays a key role in the intestinal uptake of calcium. CaT1 shows homology (75% amino acid sequence identity) to the apical calcium channel ECaC recently cloned from vitamin D-responsive cells of rabbit kidney and is structurally related to the capsaicin receptor and the TRP family of ion channels. Based on Northern analysis of rat tissues, a 3-kilobase CaT1 transcript is present in rat duodenum, proximal jejunum, cecum, and colon, and a 6.5-kilobase transcript is present in brain, thymus, and adrenal gland. lie situ hybridization revealed strong CaT1 mRNA expression in enterocytes of duodenum, proximal jejunum, and cecum, No signals were detected in kidney, heart, liver,lung, spleen, and skeletal muscle. When expressed in Xenopus oocytes, CaT1 mediates saturable Ca2+ uptake with a Michaelis constant of 0.44 mM. Transport of Ca2+ by CaT1 is electrogenic, voltage-dependent, and exhibits a charge/Ca2+ uptake ratio close to 2:1, indicating that CaT1-mediated Ca2+ influx is not coupled to other ions. CaT1 activity is pH-sensitive, exhibiting significant inhibition by low pH. CaT1 is also permeant to Sr2+ and Ba2+ (but not Mg2+). although the currents evoked by Sr2+ and Ba2+ are much smaller than those evoked by Ca2+. The trivalent cations Gd3+ and La3+ and the divalent cations Cu2+, Pb2+, Cd2+, Co2+, and Ni2+ (each at 100 mu M) do not evoke currents themselves, but inhibit CaT1-mediated Ca2+ transport. Fe3+, Fe2+, Mn2+, and Zn2+ have no significant effects at 100 mu M on CaT1-mediated Ca2+ transport. CaT1 mRNA levels are not responsive to 1,25-dihydroxyvitamin D-3 administration or to calcium deficiency. Our studies strongly suggest that CaT1 provides the principal mechanism for Ca2+ entry into enterocytes as part of the transcellular pathway of calcium absorption in the intestine.