Minireview on Regulation of Intestinal Calcium Absorption

An overview of current information on the mechanisms by which intestinal calcium absorption occurs is described in this article. Both paracellular and transcellular pathways are analyzed. Special emphasis focuses on molecules participating in the latter pathway, such as TRPV5 and TRPV6 channels, located in the apical region of the enterocytes, CB 9k and CB 28k , presumably involved in the cation movement from the apical to the basolateral pole of the cell, and PMCA 1b and Na + /Ca 2+ exchanger, proteins that extrude Ca 2+ from the cells. Current concepts on the relative importance of paracellular and transcellular calcium transport and the vitamin D dependence of each pathway are referred and analyzed showing the contrasting views on this issue. More detailed information is given regarding the stimulatory effect of vitamin D on intestinal Ca 2+ absorption either in animal models or in the human intestine. The possible mechanisms triggered by hormones such as PTH, calcitonin, estrogen, thyroid hormone, glucocorticoids and different nutritional facPublished online: February 15, 2008 Dr. Nori Tolosa de Talamoni, Laboratorio de Metabolismo Fosfocálcico ‘Dr. Fernando Cañas’, Cátedra de Bioquímica y Biología Molecular Facultad de Ciencias Médicas, Universidad Nacional de Córdoba, Pabellón Argentina 2do. Piso, Ciudad Universitaria, 5000 Córdoba (Argentina) Tel. +54 351 433 3024/int. 121, Fax +54 351 433 3072, E-Mail [email protected] © 2008 S. Karger AG, Basel 0012–2823/08/0771–0022$24.50/0 Accessible online at: www.karger.com/dig N.G.T. de T., G.P. and A.R.C. are Career Investigators of the National Council of Scientific Investigation (CONICET, Argentina). M.E.P.L. is a fellow from the Secretary of Science and Technology, National University of Córdoba, SECYT (UNC), Argentina. This work was supported by FONCYT (PICT 2005), CONICET (PIP 2005–6) and SECYT (UNC). D ow nl oa de d by : 54 .7 0. 40 .1 1 11 /1 9/ 20 17 9 :3 3: 38 P M Intestinal Ca 2+ Absorption Digestion 2008;77:22–34 23 teoporosis and other abnormalities related to the Ca 2+ metabolism. The transcellular pathway is mainly regulated by vitamin D, precisely by its hormonal metabolite 1,25(OH) 2 D 3 , via transcriptional activation of genes through binding of the ligand to the classical nuclear vitamin D receptor (VDR) [2] . Some polymorphisms of VDR such as the Fok I polymorphic site seem also to affect intestinal Ca 2+ absorption [3] by a not well-understood mechanism. Under physiological conditions, Ca 2+ ions are absorbed mainly in the small intestine, responsible for about 90% of overall Ca 2+ absorption [4] . In rat small intestine, Marcus and Lengemann [5] found that 88% of Ca 2+ absorption occurs in the ileum, 4% in the jejunum, and 8% in the duodenum. In dogs, Cramer [6] found 80, 16 and 4% in ileum, jejunum and duodenum, respectively. The longer residence time of Ca 2+ in the ileum as compared to the other intestinal segments favors Ca 2+ absorption in that segment. The transit half-time in rat ileum is around 100–120 min, whereas in the duodenum it is around 2–6 min [5, 6] . Minor amounts of Ca 2+ ions are absorbed from the stomach and large intestine; the colon accounts for less than 10% of the total Ca 2+ absorbed [4] . The major contributors to the amount of Ca 2+ absorbed are the residence time and the rate of absorption in the particular segment. The order of absorption rate is: duodenum 1 jejunum 1 ileum [7] . Colonic Ca 2+ absorption is also vitamin D-responsive and it is quite possible that it becomes important in conditions such as short bowel syndrome [8] . The bioavailability of dietary Ca 2+ affects the efficiency of intestinal Ca 2+ absorption. Low Ca 2+ diets increase the intestinal Ca 2+ absorption ratio, at least in part, by altering the vitamin D endocrine system [9, 10] and the lipid composition and fluidity of intestinal membranes [11] . Variability of intestinal Ca 2+ absorption is also related to the physiological Ca 2+ needs, but in general when the requirements increase and/or the intake is low, the efficiency of Ca 2+ absorption improves [11] . Growth, pregnancy and lactation stimulate intestinal Ca 2+ absorption, whereas aging is accompanied by a decrease in the absorption of the cation. Paracellular Pathway The intestinal epithelium is a continuous layer of individual cells with very narrow spaces between them that allow the diffusion of small molecules and ions [12] . The paracellular pathway must be regulated by the epithelium in order to maintain the selective permeability. Tight junctions constitute a barrier to the movement of molecules and ions through this pathway. These junctions are specialized membrane domains located in the apical region of enterocytes. They are intercellular structures where the plasma membranes of adjacent cells come into very close contact [12] . The proteins that form these structures are synthesized in the adjacent cells and they include occludin and another protein member of the claudin family [13] . Movement of Ca 2+ through the tight junctions is a passive process that depends on the concentration and the electric gradients across the epithelium. It is a passive non-saturable process that prevails in the jejunum and ileum, mainly when Ca 2+ intake is adequate or high [14] . It depends on the solubility of Ca 2+ in the distal small intestine, the length of sojourn of the chyme in a particular intestinal segment and the rate of diffusion from the lumen to lymph or blood [14] . When Ca 2+ intake is high, the paracellular pathway becomes important because the sojourn time in the intestine is short and the proteins involved in the transcellular route are downregulated [15] . Transcellular Pathway Epithelial Ca 2+ Channels The molecules involved in the apical Ca 2+ entry step in the intestine remained unknown until the discovery of the epithelial Ca 2+ channels TRPV5 (previously named ECaC1 or CaT2) and TRPV6 (previously named ECaC2 or CaT1) [16] . Both channels are homologous members of the transient receptor potential (TRP) superfamily, belonging precisely to the vanilloid subfamily (TRPV) to be differentiated from the canonical (TRPC) and melastatin subfamilies (TRPM). The pattern of expression of these proteins is quite variable, which can be due to differences between species or expressions below detection levels [17] . TRPV5 is the major isoform in the kidney, while TRPV6 is highly expressed in the intestine. However, TRPV5 and TRPV6 are coexpressed in human kidney and intestine, and also in other organs such as pancreas, prostate, mammary, sweat and salivary glands [16] . TRPV5 and TRPV6 have 75% homology and their main differences are located in the N and C terminal tails. Both channels permeate Ca 2+ ions, but they also permeate other divalent cations and monovalent cations in the absence of divalent ones. These channels have three unique properties: (1) they have a constitutively activated Ca 2+ permeD ow nl oa de d by : 54 .7 0. 40 .1 1 11 /1 9/ 20 17 9 :3 3: 38 P M Pérez /Picotto /Carpentieri /Rivoira / Peralta López /Tolosa de Talamoni Digestion 2008;77:22–34 24 ability; (2) the selectivity for Ca 2+ over Na + is much larger than in other members of the TRP family (P Ca /P Na 1 100), and (3) the current-voltage relationship of both channels shows inward rectification instead of outward rectification as shown by other TRPV channels. The structure of these channels is similar to that of other members of the TRP family, including 6 transmembrane domains, a short hydrophobic stretch between segments 5 and 6 which would be involved in the Ca 2+ pore and large intracellular N and C terminal tails [17] . The intracellular segments contain regulatory sites involved in the regulation of channel activity and trafficking. Among them, there are phosphorylation sites, postsynaptic density protein (zona occludens) motifs, and ankyrin repeat domains. All of them participate in the maintenance of the activity of the channels, in the interaction with other proteins, and in the targeting of those channels to specific membrane regions [17] . Hoenderop et al. [18] demonstrated that channels TRPV5 and TRPV6 have a tetrameric structure and they can combine each other to form heterotetrameric channel complexes with novel properties. The tetrameric architecture of TRPV5/6 implies that 4 aspartic residues form a ring that is negatively charged and functions as a selective filter for Ca 2+ . Although both channels originate from two genes juxtaposed on human chromosome 7q35, the proteins share several properties and have some differences. At the transcriptional level, they are regulated by 1,25(OH) 2 D 3 , estrogen and dietary Ca 2+ . Regarding the activity, both are inactivated by intracellular Ca 2+ , but the inactivation of TRPV6 shows two phases, whereas that of TRPV5 shows only a slow inactivation phase. The affinity of TRPV5 for the inhibitor ruthenium red is 100 times higher than that of TRPV6 [19] . TRPV5 and TRPV6 are located in the brush border membrane of enterocytes and it is quite possible that they are the rate-limiting entry step of active intestinal Ca 2+ absorption [20] . To determine the in vivo function of TRPV6, Bianco et al. [21] generated mice with targeted disruption of the TRPV6 gene. They have demonstrated that TRPV6 knockout (KO) mice were viable but showed a 60% decrease in intestinal Ca 2+ absorption, deficient weight gain, decreased bone mineral density (BMD), and lower fertility. Their data indicate that the TRPV6 channel not only plays a role in the tissues directly involved in Ca 2+ homeostasis, but also in other tissues. Walters et al. [22] characterized TRPV6 transcript expression in normal human intestinal biopsies. TRPV6 transcripts were detected in the duodenum but not in the ileum. Duodenal expression of TRPV6 was vitamin Ddependent in men; however, in elderly women TRPV6 and VDR expressions were reduced and were not vitamin D-dependent, which can explain, at least in part, the lower intestinal Ca 2+ absorption in elderly postmenopausal women.