Molecular Cloning and Characterization of a Channel-like Transporter Mediating Intestinal Calcium Absorption

Abstract

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.

The epithelial calcium channels TRPV5 and TRPV6: regulation and implications for disease

“…The genes of TRPV5 and TRPV6 comprise 15 exons, encoding proteins of about 730 amino acids, sharing 75% homology (Hoenderop et al 1999;Muller et al 2000b;Peng et al 1999Peng et al , 2001a. TRPV5 and TRPV6 constitute a distinct class of highly Ca 2+ -selective channels within the transient receptor potential (TRP) superfamily, which encompasses a diversity of non-voltage operated cation channels (Clapham et al 2001;Harteneck et al 2000;Montell et al 2002b).…”

“…In human, both channels are coexpressed in organs that mediate transcellular Ca 2+ transport such as duodenum, jejunum, colon, and kidney, but also in pancreas, prostate, mammary, sweat and salivary gland (Hirnet et al 2003;Hoenderop et al 1999Hoenderop et al , 2000aJanssen et al 2002;Muller et al 2000a;Peng et al 1999Peng et al , 2000Peng et al , 2001aWeber et al 2001;Wissenbach et al 2001;Zhuang et al 2002). In general, TRPV5 seems to be the major isoform in kidney, whereas TRPV6 is more ubiquitously expressed with the highest concentrations in the prostate, stomach, brain, lung and small intestine.…”

“…1), consisting of passive entry of Ca 2+ across the luminal or apical membrane, cytosolic diffusion of Ca 2+ bound to vitamin D 3 -sensitive Ca 2+ -binding proteins (calbindin-D 28K and/or calbindin-D 9K ) and active extrusion of Ca 2+ across the opposite basolateral membrane by the Na + -Ca 2+ -exchanger (NCX1) and/ or Ca 2+ -ATPase (PMCA1b; Bronner 2003;Hoenderop et al 2002b). The molecular identity of the apical Ca 2+ entry pathway remained elusive until the epithelial Ca 2+ channels TRPV5 (previously named ECaC1; Hoenderop et al 1999) and TRPV6 (previously named Ca 2+ transporter 1) were identified (Montell et al 2002b;Peng et al 1999). These channels convey the rate-limiting step in active Ca 2+ transport and play, therefore, a pivotal role in Ca 2+ homeostasis.…”

Opening of the human epithelial calcium channel TRPV6

“… Ca 2+ -selective transient receptor potential vanilloid subfamily member 6 (TRPV6) channels play a critical role in calcium uptake in epithelial tissues 1–4 . Altered TRPV6 expression is associated with a variety of human diseases 5 , including cancers 6 .…”

“…Altered TRPV6 expression is associated with a variety of human diseases 5 , including cancers 6 . TRPV6 channels are constitutively active 1,7,8 and their open probability depends on the lipidic composition of the membrane, increasing significantly in the presence of phosphatidylinositol 4,5-bisphosphate (PIP 2 ) 7,9 . We previously solved crystal structures of detergent-solubilized rat TRPV6 in the closed state 10,11 .…”