Molecular Identification of the Apical Ca2+Channel in 1,25-Dihydroxyvitamin D3-responsive Epithelia

Abstract

In mammals, the extracellular calcium concentration is maintained within a narrow range despite large variations in daily dietary input and body demand. The small intestine and kidney constitute the influx pathways into the extracellular Ca 2؉ pool and, therefore, play a primary role in Ca 2؉ homeostasis. We identified an apical Ca 2؉ influx channel, which is expressed in proximal small intestine, the distal part of the nephron and placenta. This novel epithelial Ca 2؉ channel (ECaC) of 730 amino acids contains six putative membranespanning domains with an additional hydrophobic stretch predicted to be the pore region. ECaC resembles the recently cloned capsaicin receptor and the transient receptor potential-related ion channels with respect to its predicted topology but shares less than 30% sequence homology with these channels. In kidney, ECaC is abundantly present in the apical membrane of Ca 2؉ transporting cells and colocalizes with 1,25-dihydroxyvitamin D 3 -dependent calbindin-D 28K . ECaC expression in Xenopus oocytes confers Ca 2؉ influx with properties identical to those observed in distal renal cells. Thus, ECaC has the expected properties for being the gatekeeper of 1,25-dihydroxyvitamin D 3 -dependent active transepithelial Ca 2؉ transport.

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.…”

The Transient Receptor Potential Superfamily of Ion Channels

“…Transcellular reabsorption of Ca 2ϩ and Mg 2ϩ is mediated by diffusion into cells through ion channels in the apical membrane followed by extrusion across basolateral membranes via pumps or exchangers. TRPV5 was initially identified as the apical Ca 2ϩ entry channel in kidney and was localized to the Ca 2ϩ -reabsorptive distal convoluted tubules (DCT) (17). TRPV6 was identified as the apical channel responsible for intestinal absorption of Ca 2ϩ (18).…”

“…TRPV1 to 4 share 40 to 50% amino acid homology, are Ca 2ϩ -permeable nonselective cation channels (P Ca /P Na~3 to 10:1), and have steep temperature sensitivity. TRPV5 and TRPV6 were isolated by expression cloning of proteins that mediate Ca 2ϩ transport in kidney and intestine, respectively (17,18). They are the most highly Ca 2ϩ -selective TRP channels (P Ca /P Na Ͼ100:1).…”