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TRPV6
Transcellular
Paracellular transport
Jejunum
TRPV6
Transcellular
Paracellular transport
Jejunum
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Intestinal cells are continuously exposed to food whose components are able to modulate some of their physiological functions. Among the bioactive food derivatives are casein phosphopeptides (CPPs), coming from the in vitro or in vivo casein digestion, which display the ability to form aggregates with calcium ions and to increase the uptake of the minerals in differentiated intestinal human HT-29 and Caco2 cells. Since extracellular calcium is a known inactivator of the TRPV6 channel, which is also involved in the colon cancer progression, the present study aims to determine a possible modulation by CPPs of the molecular structures responsible for paracellular and/or transcellular calcium absorption in these two cell lines. The paracellular calcium transport was determined by TEER measurements in Caco2 cells and by Lucifer Yellow flow in HT-29 cells. The possible modulation of transcellular calcium absorption machinery by CPPs was investigated by determining the mRNA expression for both the TRPV6 calcium channel and the VDR receptor in 1,25(OH)2D3 pre-treated undifferentiated/differentiated cells. The results obtained point out that: (i) CPPs do not affect paracellular calcium absorption; (ii) 1,25(OH)2D3 increases the TRPV6 mRNA expression in both types of cells. In the case of HT-29 cells this is the first determination of the presence of the TRPV6 channel; (iii) CPPs per se are not able to affect the VDR and TRPV6 mRNA expression; (iv) CPP administration does not affect the TRPV6 mRNA expression in 1,25(OH)2D3 pre-treated HT-29 cells and Caco2 cells. Unlike peptides coming from the digestion of cheese whey protein digest, the digestion of milk casein produces peptides with no effects on TRPV6 calcium channel expression, though the same peptides are able to determine a calcium uptake by the intestinal cells.
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TRPV6
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Caco-2
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Paracellular transport
Transcellular
TRPV6
Ussing chamber
Homeostasis
Calcium ATPase
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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, CB9k and CB28k, presumably involved in the cation movement from the apical to the basolateral pole of the cell, and PMCA1b and Na+/Ca2+ exchanger, proteins that extrude Ca2+ 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 Ca2+ 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 factors on intestinal calcium absorption are also reviewed. Finally, the influence of physiological conditions such as growth, pregnancy, lactation and aging on intestinal calcium absorption are discussed.
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TRPV6
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Paracellular transport
Transcellular
TRPV6
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Paracellular transport
Transcellular
Jejunum
TRPV6
Intestinal mucosa
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alcium is the fifth most abundant element in the earth’s crust and the most abundant cation in the human body. A 70-kg person possesses roughly 1 kg of calcium. Of this, 99% is in the mineral phase of the bones and teeth and 1% is in the extracellular and intracellular fluids. The ionic form of calcium serves as a universal intracellular messenger to modulate many processes, such as neurotransmission, muscle contraction, and secretion. Since all of the calcium in our bodies is ultimately absorbed from the diet, intestinal calcium absorption is an important determinant of calcium homeostasis. Two pathways are responsible for calcium entry into the body: the paracellular and the transcellular pathways. In the paracellular pathway, calcium enters through tight junctions located between the epithelial cells, whereas in the transcellular pathway, calcium enters across the apical and basolateral membranes of a cell, a process that requires an apical calcium entry channel, an intracellular calcium binding protein called calbindin (calbindin D 9k ), and calcium pump [plasma membrane calcium ATPase (PMCA 1b )]. Similar mechanisms of transcellular calcium transport exist in the renal distal tubules and the syncytium of the placenta. Recently, our understanding of the paracellular and transcellular pathways has been advanced by the identification of the channels for both pathways. With respect to the paracellular pathway, claudins are thought to form part of a paracellular channel-like structure. In the present review, we will focus on two recently identified apical calcium channels, calcium transport protein subtype 1 (CaT1) and epithelial calcium channel (ECaC), in the transcellular pathway. (Basic information about these two channels is available in Table 1.) CaT1 was first identified in the rat intestine (12) and ECaC in the rabbit kidney (4). Several other names (CaT-L/ECaC2 and CaT2/ECaC1) were also used for CaT1 and ECaC, respectively (Table 1). Recently, a human gene nomenclature was introduced for cation channels related to the Drosophila transient receptor potential (TRP) channels. Since CaT1 and ECaC are distally related to the TRPs, the human genes for CaT1 and ECaC were termed TRPV6 and TRPV5, respectively [“V” denotes “vanilloid receptor (VR1)-related”]. The human CaT1 and ECaC share 75% amino acid sequence identity. They are encoded by two genes juxtaposed on human chromosome 7q3335. One is likely duplicated from the other during the course of evolution. CaT1 and ECaC are calcium-selective members of a cation channel subfamily consisting of six members (10). The other four members of the family encode nonselective cation channels that serve as sensors. Three of them (VR1, VRL-1, and TRPV3) are heat sensors that are expressed in sensory neurons and skin, and another one (OTRPC4/VR-OAC/VRL-2/TRP12) is an osmoreceptor expressed in the central nervous system and the kidney. In contrast, CaT1 and ECaC, which share ~30% amino acid identity with the other channels of the family, serve as apical transporters in calcium-transporting epithelia.
Paracellular transport
Transcellular
TRPV6
Calcium in biology
Apical membrane
Calcium ATPase
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Transcellular
Paracellular transport
TRPV6
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Paracellular transport
Transcellular
Enterocyte
TRPV6
Intestinal epithelium
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The placenta has many essential roles in the maintenance of pregnancy and homeostasis. Calcium transport and regulation are also controlled by the placenta. In general, calcium transport is divided into an active transcellular pathway and a passive paracellular pathway. Transient receptor potential cation channel subfamily V member 5/6 (TRPV5/6), calbindin-D9k/-28k (CaBP-9k/-28k) and Na+/Ca2+ exchanger (NCX1) are involved in the transcellular pathway. The paracellular pathway is determined by the expression of tight junction genes, such as occludin, claudins and ZO-1. In this study, we analysed the difference in calcium transport in the placentae of CaBP-9k and CaBP-28k knockout (KO) mice compared with that of wild-type (WT) mice. Placentae were collected and used for mRNA and protein evaluation from 9 mice of each type (a total of 36 mice). All mice were killed on gestational Day 19. We confirmed mRNA expression by RT-qPCR and protein expression by Western blot analysis. The data were statistically analysed by one-way ANOVA using Tukey's test. In the transcellular pathway, the expression levels of NCX1 and TPRV6 were shown to be significantly increased in KO mice compared with WT mice. In the paracellular pathway, occluding, which is directly related to permeability, mRNA and protein expression was significantly increased in single KO mice compared with WT, but not in double KO mice. Claudin-4, which is a cation barrier, mRNA and protein expression patterns were significantly decreased in single KO mice compared with WT, but not in double KO mice. These results imply that the disability of calcium buffering due to CaBP-9k or CaBP-28k KO may lead to an accelerated transcellular pathway. In addition, a single KO of calcium-binding proteins may lead to decreased paracellular permeability to weaken the leakage of calcium through the tight junction and may also lead to increased cation selectivity of tight junctions. Taken together, these results might indicate that KO of calcium-binding proteins may induce activation of a compensating transcellular pathway in the placenta of mice and a paracellular pathway may support the maintenance of calcium homeostasis.
Paracellular transport
Transcellular
Claudin
TRPV6
Occludin
Knockout mouse
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