A rat kidney-specific calcium transporter in the distal nephron

Acute study of interaction among cadmium, calcium, and zinc transport along the rat nephron in vivo

This study investigates the effect in rats of acute CdCl(2) (5 microM) intoxication on renal function and characterizes the transport of Ca(2+), Cd(2+), and Zn(2+) in the proximal tubule (PT), loop of Henle (LH), and terminal segments of the nephron (DT) using whole kidney clearance and nephron microinjection techniques. Acute Cd(2+) injection resulted in renal losses of Na(+), K(+), Ca(2+), Mg(2+), PO(4)(-2), and water, but the glomerular filtration rate remained stable. (45)Ca microinjections showed that Ca(2+) permeability in the DT was strongly inhibited by Cd(2+) (20 microM), Gd(3+) (100 microM), and La(3+) (1 mM), whereas nifedipine (20 microM) had no effect. (109)Cd and (65)Zn(2+) microinjections showed that each segment of nephron was permeable to these metals. In the PT, 95% of injected amounts of (109)Cd were taken up. (109)Cd fluxes were inhibited by Gd(3+) (90 microM), Co(2+) (100 microM), and Fe(2+) (100 microM) in all nephron segments. Bumetanide (50 microM) only inhibited (109)Cd fluxes in LH; Zn(2+) (50 and 500 microM) inhibited transport of (109)Cd in DT. In conclusion, these results indicate that 1) the renal effects of acute Cd(2+) intoxication are suggestive of proximal tubulopathy; 2) Cd(2+) inhibits Ca(2+) reabsorption possibly through the epithelial Ca(2+) channel in the DT, and this blockade could account for the hypercalciuria associated with Cd(2+) intoxication; 3) the PT is the major site of Cd(2+) reabsorption; 4) the paracellular pathway and DMT1 could be involved in Cd(2+) reabsorption along the LH; 5) DMT1 may be one of the major transporters of Cd(2+) in the DT; and 6) Zn(2+) is taken up along each part of the nephron and its transport in the terminal segments could occur via DMT1.

Aldosterone enhances renal calcium reabsorption by two types of channels

BACKGROUND

Aldosterone has been known for many years to increase sodium (Na(+)) reabsorption by the distal nephron. The present in vitro experiments investigated the effect of the hormone on calcium (Ca(2+)) transport by the luminal membrane of the rabbit nephron, independent of any systemic influence.

METHODS

Proximal and distal tubules were incubated with either aldosterone or the carrier. The luminal membranes of these tubules were purified, vesiculated, and (45)Ca uptake by these vesicles was subsequently measured.

RESULTS

Treatment of the distal tubules with 10(-8) mol/L aldosterone enhanced both 0.1 and 0.5 mmol/L Ca(2+) transport. The hormone action was abolished by tyrosine kinase inhibitors. The presence of Na(+) in the medium decreased both Ca(2+) uptake and the effect of aldosterone. This hormone action was already significant after a 5-minute incubation, with a half-maximal efficient concentration of approximately 10(-10) mol/L. Ca(2+) transport by the distal membranes presents a dual kinetics. Aldosterone enhanced the Vmax values of both components of these kinetics. Mibefradil abolished the action of aldosterone on 0.5 mmol/L but not on 0.1 mmol/L Ca(2+) uptake, suggesting that the targeted low affinity channel belongs to the T-type, whereas diltiazem prevented the hormone action exclusively at the low Ca(2+) concentration (0.1 mmol/L), indicating an effect on a high affinity L-type channel.

CONCLUSION

Aldosterone increases Ca(2+) transport by the distal luminal membranes through L- and T-type Ca(2+) channels, and this action requires tyrosine kinase activity.

Biological actions and mechanism of action of calbindin in the process of apoptosis

Although it was originally proposed that the major role of calbindin is to facilitate the vitamin D dependent movement of calcium through the cytosolic compartment of the intestinal or renal cell, we found that calbindin also has a major role in different cell types in protecting against apoptotic cell death. Calbindin, which buffers calcium, can inhibit apoptosis induced by different proapoptotic stimuli. Expression of calbindin-D(28k) in neural cell suppressed the proapoptotic actions of presenilin-1, which is causally linked to familial Alzheimer's disease, by preventing calcium mediated mitochondrial damage and the subsequent release of cytochrome c. Calbindin, by buffering intracellular calcium can also protect HEK 293 kidney cells from parathyroid hormone induced apoptosis that was found to be mediated by a phospholipase C dependent increase in intracellular calcium. In addition, cytokine mediated destruction of pancreatic beta cells can be prevented by calbindin. Induction by cytokines of nitric oxide, peroxynitrite and lipid hydroperoxide production was significantly decreased in calbindin expressing beta cells. Thus, calbindin-D(28k), by inhibiting free radical formation, can protect islet beta cells from autoimmune destruction in type 1 diabetes. Calbindin-D(28k) can also protect against apoptosis in bone cells. Calbindin was found to block apoptosis in osteocytic and osteoblastic cells. Our findings suggest that calbindin is capable of directly inhibiting the activity of caspase-3, a common downstream effector of multiple apoptotic signaling pathways, and that this inhibition results in an inhibition of tumor necrosis factor (TNFalpha) and glucocorticoid induced apoptosis in bone cells. Thus, while part of calbindin's protective effect may result from buffering rises in intracellular calcium, other mechanisms of action, such as inhibition of caspase activity, also play a significant role in the prevention of apoptosis by calbindin-D(28k). These findings have implications for the prevention of degeneration in different cell types and therefore could prove important for the therapeutic intervention of many diseases, including diabetes and osteoporosis.

Regulation of gene expression of epithelial calcium channels in intestine and kidney of mice by 1alpha,25-dihydroxyvitamin D3

In wild-type (VDR(+/+)) mice, ECaC2 expression was confirmed in the intestine and kidney, while ECaC1 expression was exclusively confined to the kidney. Both mRNAs expression of ECaC1 and ECaC2 in the kidney and ECaC2 mRNA expression in the intestine increased time- and dose-dependently in response to 1alpha,25(OH)(2)D(3) injection in VDR(+/+) mice, but not in VDR(-/-) mice. The mRNA levels of ECaC2 in the intestine of VDR(-/-) mice were remarkably reduced when compared to VDR(+/+) mice, while no significant differences were observed in both mRNA levels of ECaC1 and ECaC2 in the kidney between VDR(+/+) mice and VDR(-/-) mice. In the primary renal tubular cells (PRTC) isolated from VDR(+/+) mice, both ECaCs mRNA expression increased in response to 1alpha,25(OH)(2)D(3) treatment, but not in the PRTC of VDR(-/-) mice. PTH increased both ECaCs mRNA expression in the PRTC of VDR(+/+) mice. These results suggest that 1alpha,25(OH)(2)D(3) directly modulates the gene expression of ECaC1 and ECaC2 together with PTH in the kidney of mice. 1alpha,25(OH)(2)D(3) also modulates the gene expression of ECaC2 in the intestine of mice, however, further studies are needed to elucidate the direct action of 1alpha,25(OH)(2)D(3) on the expression of ECaC2 in the intestine.

Calcium-sensing receptor regulation of renal mineral ion transport

Extracellular calcium has long been known to affect the rate and magnitude of renal calcium and phosphate recovery. In this review, we consider some of these findings in light of our present understanding of the tubular localization of the calcium-sensing receptor (CaSR). Experiments directly implicating the CaSR in regulating calcium and phosphate transport are described. These results point to an important role of the CaSR in regulating PTH-dependent calcium absorption by cortical thick ascending limbs and on PTH-sensitive proximal tubule phosphate transport. Possible avenues for further investigation are suggested.

TRPV4 calcium entry channel: a paradigm for gating diversity

The vanilloid receptor-1 (VR1, now TRPV1) was the founding member of a subgroup of cation channels within the TRP family. The TRPV subgroup contains six mammalian members, which all function as Ca2+ entry channels gated by a variety of physical and chemical stimuli. TRPV4, which displays 45% sequence identity with TRPV1, is characterized by a surprising gating promiscuity: it is activated by hypotonic cell swelling, heat, synthetic 4alpha-phorbols, and several endogenous substances including arachidonic acid (AA), the endocannabinoids anandamide and 2-AG, and cytochrome P-450 metabolites of AA, such as epoxyeicosatrienoic acids. This review summarizes data on TRPV4 as a paradigm of gating diversity in this subfamily of Ca2+ entry channels.

Hereditary disorders of potassium homeostasis

There has been a dramatic recent increase in the understanding of the renal epithelial transport systems with the identification, cloning and characterization of a large number of membrane transport proteins. The aim of this chapter is to integrate this body of knowledge with the understanding of the clinical disorders that accompany gain, loss or dysregulation of function of these transport systems. The specific focus is on the best-defined human clinical syndromes in which there are derangements in potassium (K(+)) homeostasis. The focus is on inherited syndromes, rather than on acquired syndromes due to tubular transport defects, and the therapeutic approaches address chronic derangements of K(+) homeostasis rather than acute interventions directed at life-threatening hyperkalaemia.

Mechanism and Molecular Determinant for Regulation of Rabbit Transient Receptor Potential Type 5 (TRPV5) Channel by Extracellular pH

The transient receptor potential type 5 (TRPV5) channel is present in kidney and intestine and important for transepithelial (re)absorption of calcium in these tissues. We report that in whole-cell patch clamp recording extracellular acidification inhibited rabbit TRPV5 with apparent pKa approximately 6.55. The two extracellular loops between the fifth and sixth transmembrane segments of TRPV5 presumably form part of the outer opening of the pore and likely are important in binding and regulation by external protons. We found that mutation of glutamate 522 to glutamine (E522Q) decreased the sensitivity of the channel to extracellular acidification. Mutations of other titratable amino acids within the two extracellular loops to non-titratable amino acids had no effect on pH sensitivity. Substitutions of aspartate or other titratable amino acids for glutamate 522 conferred an increase in pH sensitivity. The pH sensitivity mediated by glutamate 522 was independent of extracellular or intracellular Mg2+. Single channel analysis revealed that extracellular acidification reduced single channel conductance as well as open probability of the wild type channel. In contrast to wild type channel, extracellular acidification did not reduce open probability for E522Q mutant. Methanethiosulfonate reagents inhibited the activity of glutamine 522 to cysteine mutant channel with a reaction rate constant approaching that with free thiols in solution, suggesting that glutamate 522 is located on the surface of the channel. These data suggest that glutamate 522 of the rabbit TRPV5 is a "pH sensor," and extracellular protons inhibit TRPV5 likely by altering conformation of the channel protein.

Outer pore topology of the ECaC-TRPV5 channel by cysteine scan mutagenesis

The substituted cysteine accessibility method (SCAM) was used to map the external vestibule and the pore region of the ECaC-TRPV5 calcium-selective channel. Cysteine residues were introduced at 44 positions from the end of S5 (Glu515) to the beginning of S6 (Ala560). Covalent modification by positively charged MTSET applied from the external medium significantly inhibited whole cell currents at 15/44 positions. Strongest inhibition was observed in the S5-linker to pore region (L520C, G521C, and E522C) with either MTSET or MTSES suggesting that these residues were accessible from the external medium. In contrast, the pattern of covalent modification by MTSET for residues between Pro527 and Ile541 was compatible with the presence of a alpha-helix. The absence of modification by the negatively charged MTSES in that region suggests that the pore region has been optimized to favor the entrance of positively charged ions. Cysteine mutants at positions -1, 0, +1, +2 around Asp542 (high Ca2+ affinity site) were non-functional. Whole cell currents of cysteine mutants at +4 and +5 positions were however covalently inhibited by external MTSET and MTSES. Altogether, the pattern of covalent modification by MTS reagents globally supports a KcsA homology-based three-dimensional model whereby the external vestibule in ECaC-TRPV5 encompasses three structural domains consisting of a coiled structure (Glu515 to Tyr526) connected to a small helical segment of 15 amino acids (527PTALFSTFELFLT539) followed by two distinct coiled structures Ile540-Pro544 (selectivity filter) and Ala545-Ile557 before the beginning of S6.

Epithelial Ca2+ entry channels: transcellular Ca2+ transport and beyond

The recently discovered apical calcium channels CaT1 (TRPV6) and ECaC (TRPV5) belong to a family of six members called the 'TRPV family'. Unlike the other four members which are nonselective cation channels functioning as heat or osmolarity sensors in the body, CaT1 and ECaC are remarkably calcium-selective channels which serve as apical calcium entry mechanisms in absorptive and secretory tissues. CaT1 is highly expressed in the proximal intestine, placenta and exocrine tissues, whereas ECaC expression is most prominent in the distal convoluted and connecting tubules of the kidney. CaT1 in the intestine is highly responsive to 1,25-dihydroxyvitamin D3 and shows both fast and slow calcium-dependent feedback inhibition to prevent calcium overload. In contrast, ECaC only shows slow inactivation kinetics and appears to be mostly regulated by the calcium load in the kidney. Outside the calcium-transporting epithelia, CaT1 is highly expressed in exocrine tissues such as pancreas, prostate and salivary gland. In these tissues it probably mediates re-uptake of calcium following its release by secretory vesicles. CaT1 also contributes to store-operated calcium entry in Jurkat T-lymphocytes and prostate cancer LNCaP cells, possibly in conjunction with other cellular components which link CaT1 activity to the filling state of the calcium stores. Finally, CaT1 expression is upregulated in prostate cancer and other cancers of epithelial origin, highlighting its potential as a target for cancer therapy.

Apical entry channels in calcium-transporting epithelia

The identification of the apical calcium channels CaT1 and ECaC revealed the key molecular mechanisms underlying apical calcium entry in calcium-transporting epithelia. These channels are regulated directly or indirectly by vitamin D and dietary calcium and undergo feedback control by intracellular calcium, suggesting their rate-limiting roles in transcellular calcium transport.

The epithelial calcium channels, TRPV5 & TRPV6: from identification towards regulation

The epithelial calcium channels, TRPV5 and TRPV6, have been extensively studied in epithelial tissues controlling the Ca(2+) homeostasis and exhibit a range of distinctive properties that distinguish them from other TRP channels. This review focuses on the tissue distribution, the functional properties, the architecture and the regulation of the expression and activity of the TRPV5 and TRPV6 channel.

The calcium-sensing receptor is required for normal calcium homeostasis independent of parathyroid hormone

The extracellular calcium-sensing receptor (CaR; alternate gene names, CaR or Casr) is a membrane-spanning G protein-coupled receptor. CaR is highly expressed in the parathyroid gland, and is activated by extracellular calcium (Ca(2+)(o)). Mice homozygous for null mutations in the CaR gene (CaR(-/-)) die shortly after birth because of the effects of severe hyperparathyroidism and hypercalcemia. A wide variety of functions have been attributed to CaR. However, the lethal CaR-deficient phenotype has made it difficult to dissect the direct effect of CaR deficiency from the secondary effects of hyperparathyroidism and hypercalcemia. We therefore generated parathyroid hormone-deficient (PTH-deficient) CaR(-/-) mice (Pth(-/-)CaR(-/-)) by intercrossing mice heterozygous for the null CaR allele with mice heterozygous for a null Pth allele. We show that genetic ablation of PTH is sufficient to rescue the lethal CaR(-/-) phenotype. Pth(-/-)CaR(-/-) mice survive to adulthood with no obvious difference in size or appearance relative to control Pth(-/-) littermates. Histologic examination of most organs did not reveal abnormalities. These Pth(-/-)CaR(-/-) mice exhibit a much wider range of values for serum calcium and renal excretion of calcium than we observe in control littermates, despite the absence of any circulating PTH. Thus, CaR is necessary for the fine regulation of serum calcium levels and renal calcium excretion independent of its effect on PTH secretion.

Sodium and calcium transport pathways along the mammalian distal nephron: from rabbit to human

The final adjustment of renal sodium and calcium excretion is achieved by the distal nephron, in which transepithelial ion transport is under control of various hormones, tubular fluid composition, and flow rate. Acquired or inherited diseases leading to deranged renal sodium and calcium balance have been linked to dysfunction of the distal nephron. Diuretic drugs elicit their effects on sodium balance by specifically inhibiting sodium transport proteins in the apical plasma membrane of distal nephron segments. The identification of the major apical sodium transport proteins allows study of their precise distribution pattern along the distal nephron and helps address their cellular and molecular regulation under various physiological and pathophysiological settings. This review focuses on the topological arrangement of sodium and calcium transport proteins along the cortical distal nephron and on some aspects of their functional regulation. The availability of data on the distribution of transporters in various species points to the strengths, as well as to the limitations, of animal models for the extrapolation to humans.

New insights into the mechanisms of vitamin D action

The biologically active metabolite of vitamin D, 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) is a secosteroid whose genomic mechanism of action is similar to that of other steroid hormones and is mediated by stereospecific interaction of 1,25(OH)(2)D(3) with the vitamin D receptor (VDR) which heterodimerizes with the retinoid X receptor (RXR). After interaction with the vitamin D response element (VDRE) in the promoter of target genes, transcription proceeds through the interaction of VDR with coactivators and with the transcription machinery. The identification of the steps involved in this process has been a major focus of recent research in the field. However, the functional significance of target proteins as well as the functional significance of proteins involved in the transport and metabolism of vitamin D is also of major importance. Within the past few years much new information has been obtained from studies using knockout and transgenic mice. New insight has been obtained using this technology related to the physiological significance of the vitamin D binding protein (DBP), used to transport vitamin D metabolites, as well as the physiological significance of target proteins including 25-hydroxyvitamin D(3) 24-hydroxylase (24(OH)ase), 25-hydroxyvitamin D(3)-1 alpha-hydroxylase (1 alpha-(OH)ase), VDR, and osteopontin. The crystal structure of the DBP and the ligand binding domain of the VDR have recently been reported, explaining, in part, the unique properties of these proteins. In addition novel 1,25(OH)(2)D(3) target genes have been identified including the epithelial calcium channel, present in the proximal intestine and in the distal nephron. Thus in recent years a number of exciting discoveries have been made that have enhanced our understanding of mechanisms involved in the pleiotropic actions of 1,25(OH)(2)D(3).

Vitamin D target proteins: function and regulation

Recent findings have indicated that calbindin-D(28k), the first known target of vitamin D action, is present in osteoblasts and protects against TNF and glucocorticoid induced apoptosis of osteoblastic cells. Cytokine mediated destruction of pancreatic beta cells, a cause of insulin dependent diabetes, is also inhibited by calbindin-D(28k). In calbindin-D(28k) transfected pancreatic beta cells free radical formation by cytokines is inhibited by calbindin. Thus, besides its role as a facilitator of calcium diffusion, calbindin has a major role in protecting against cellular degeneration in different cell types. Besides calbindin, the other known pronounced effect of 1,25(OH)(2)D(3) in intestine and kidney is increased synthesis of 25(OH)D(3) 24-hydroxylase (24(OH)ase) which is involved in the catabolism of 1,25(OH)(2)D(3). We have noted that CCAAT enhancer binding protein beta (C/EBPbeta) is induced by 1,25(OH)(2)D(3) in kidney and osteoblastic cells and can enhance the transcriptional response of 24(OH)ase to 1,25(OH)(2)D(3). These studies establish C/EBPbeta as a novel 1,25(OH)(2)D(3) target gene and indicate a role for C/EBPbeta in 24(OH)ase transcription. These studies extend our previous studies related to factors that affect vitamin D receptor (VDR) mediated 24(OH)ase transcription (YY1, TFIIB, CBP) and the effect of signaling pathways on 24(OH)ase transcription and cofactor recruitment.

Intestinal calcium absorption: Molecular vitamin D mediated mechanisms

Rickets and hyperparathyroidism caused by a defective Vitamin D receptor (VDR) can be prevented in humans and animals by high calcium intake, suggesting that intestinal calcium absorption is critical for 1,25(OH)(2) vitamin D [1,25-(OH)(2)D(3)] action on calcium homeostasis. We assessed the rate of serum (45)Ca accumulation within 10 min after oral gavage in two strains of VDR-knock out (KO) mice (Leuven and Tokyo KO) and observed a threefold lower area under the curve in both KO-strains. Moreover, we evaluated the expression of intestinal candidate genes, belonging to a new class of calcium channels (TRPV), involved in transcellular calcium transport. The calcium transport protein ECaC2 was more abundantly expressed at mRNA level than ECaC1 in duodenum, but both were considerably reduced (ECaC2 > 90%, ECaC1 > 60%) in the two VDR-KO strains on a normal calcium diet. Calbindin-D(9K) expression was only significantly decreased in the Tokyo KO, whereas PMCA(1b) expression was normal in both VDR-KOs. In Leuven wild type mice, a high calcium diet inhibited (> 90%), and 1,25(OH)(2)D(3) or low calcium diet induced (sixfold) duodenal ECaC2 expression and, to a lesser degree, ECaC1 and calbindin-D(9K) expression. In Leuven KO mice, however, high or low calcium intake decreased calbindin-D(9K) and PMCA(1b) expression, whereas both ECaC mRNA expressions remained consistently low on any diet. These results suggest that the expression of the novel duodenal epithelial calcium channels (in particular ECaC2 or TRPV6) is strongly vitamin D dependent and that calcium influx, probably interacting with calbindin-D(9K), should be considered as a rate-limiting step in the process of vitamin D dependent active calcium absorption.

Calcium fluxes in human trophoblast (BeWo) cells: calcium channels, calcium-ATPase, and sodium-calcium exchanger expression

Although placental transfer of maternal calcium (Ca(2+)) is a crucial process for fetal development, the biochemical mechanisms are poorly understood. In the current study, we have investigated the characteristics of Ca(2+) fluxes in relation with cell Ca(2+) homeostasis in the human placental trophoblast cell line BeWo. Time-courses of Ca(2+) uptake by BeWo cells displayed rapid initial entry (initial velocity (V(i)) of 3.42 +/- 0.35 nmol/mg protein/min) and subsequent establishment of a plateau. Ca(2+) efflux studies with (45)Ca(2+)-loaded cells also showed rapid declined of cell-associated (45)Ca(2+) with a V(i) of efflux (Ve(i)) of 3.30 +/- 0.08 nmol/mg protein/min. Further identification of membrane gates for Ca(2+) entry in BeWo cells was carried out. Expression of Ca(2+) transporter/channel CaT1 and L-type alpha(1S) subunit was showed by RT-PCR. However, mRNA for CaT2 channel and L-type alpha(1C) and alpha(1D) subunits were not revealed. Membrane systems responsible for intracellular Ca(2+) extrusion from BeWo cells were also investigated. Plasma membrane Ca(2+)-ATPases (PMCA) and Na/Ca exchangers (NCX) were detected by Western blot in BeWo cells. Expression of specific isoforms of PMCA and NCX was further investigated by RT-PCR. Messenger RNAs of four isoforms of PMCA (PMCA 1-4) were detected. The presence of messenger RNAs of two NCX isoforms (NCX1 and NCX3) was observed. Ca(2+) flux studies in Na-free incubation medium indicated that NCX played a minimal role in the cell Ca(2+) fluxes. Inorganic ions such as cadmium and manganese did not modify the Ca(2+) fluxes, however, barium increased cell-associated (45)Ca(2+) by, in part, by reducing radiolabel exit.

TRPM7 provides an ion channel mechanism for cellular entry of trace metal ions

Trace metal ions such as Zn(2+), Fe(2+), Cu(2+), Mn(2+), and Co(2+) are required cofactors for many essential cellular enzymes, yet little is known about the mechanisms through which they enter into cells. We have shown previously that the widely expressed ion channel TRPM7 (LTRPC7, ChaK1, TRP-PLIK) functions as a Ca(2+)- and Mg(2+)-permeable cation channel, whose activity is regulated by intracellular Mg(2+) and Mg(2+).ATP and have designated native TRPM7-mediated currents as magnesium-nucleotide-regulated metal ion currents (MagNuM). Here we report that heterologously overexpressed TRPM7 in HEK-293 cells conducts a range of essential and toxic divalent metal ions with strong preference for Zn(2+) and Ni(2+), which both permeate TRPM7 up to four times better than Ca(2+). Similarly, native MagNuM currents are also able to support Zn(2+) entry. Furthermore, TRPM7 allows other essential metals such as Mn(2+) and Co(2+) to permeate, and permits significant entry of nonphysiologic or toxic metals such as Cd(2+), Ba(2+), and Sr(2+). Equimolar replacement studies substituting 10 mM Ca(2+) with the respective divalent ions reveal a unique permeation profile for TRPM7 with a permeability sequence of Zn(2+) approximately Ni(2+) >> Ba(2+) > Co(2+) > Mg(2+) >/= Mn(2+) >/= Sr(2+) >/= Cd(2+) >/= Ca(2+), while trivalent ions such as La(3+) and Gd(3+) are not measurably permeable. With the exception of Mg(2+), which exerts strong negative feedback from the intracellular side of the pore, this sequence is faithfully maintained when isotonic solutions of these divalent cations are used. Fura-2 quenching experiments with Mn(2+), Co(2+), or Ni(2+) suggest that these can be transported by TRPM7 in the presence of physiological levels of Ca(2+) and Mg(2+), suggesting that TRPM7 represents a novel ion-channel mechanism for cellular metal ion entry into vertebrate cells.

Calcium-selective ion channel, CaT1, is apically localized in gastrointestinal tract epithelia and is aberrantly expressed in human malignancies

CaT1 is a highly selective calcium entry channel that has been proposed to be responsible for apical calcium entry in the vitamin D-regulated transcellular pathway of Ca(2+) absorption; however, the lack of a CaT1 antibody suitable for immunohistochemistry has prevented the direct testing of this hypothesis by the localization of CaT1 protein in the gastrointestinal tract and other tissues. In this study, we developed two CaT1 antibodies and have used them to establish for the first time that CaT1 localizes to the apical membrane of intestinal absorptive cells, thereby providing the first direct evidence that this protein is in fact an apical entry channel in the gastrointestinal tract. In addition, we found that CaT1 protein is highly expressed in a number of exocrine organs including pancreas, prostate, and mammary gland, suggesting an, as yet, unrecognized role in secretory epithelia. Finally, we found CaT1 protein to be present at elevated levels in comparison with normal tissues in a series of prostate, breast, thyroid, colon, and ovarian carcinomas, consistent with previous reports of up-regulation of CaT1 mRNA in prostate cancer tissues. Our findings indicate that CaT1 is likely to serve as a component of transcellular calcium transport mechanisms in many tissues and epithelial cancers.

Cyclosporine A-induced hypercalciuria in calbindin-D28k knockout and wild-type mice

BACKGROUND

It is known that cyclosporine A (CsA) treatment induces high bone-turnover osteopenia and hypercalciuria. It has been proposed that down-regulation of renal calbindin-D28k by CsA results in renal calcium wasting. We investigated the role of the kidney and bone in CsA-induced hypercalciuria in calbindin-D28k knockout (KO) and wild-type (WT) mice.

METHODS

Two sets of experiments were performed. In experiment 1, KO and WT mice were treated with CsA 20 mg/kg/day intraperitoneally (IP) for 7 days. In experiment 2, to eliminate the CsA effect on bone resorption, pamidronate (APD) 2.5 mg/kg IP was given every 4 days with the first dose given 4 days prior to the 7-day course of CsA. Serum levels of creatinine, calcium, and osteocalcin, as well as renal calcium excretion were measured to assess CsA's effects on calcium homeostasis. Effects of CsA on the expression of calbindin-D28k, and two calcium channels in the apical membrane of the distal tubule, epithelial calcium channel (ECaC) and alpha1G-subunit of a voltage-dependent Ca channel (alpha1G), in the kidney were examined by semiquantitative reverse transcription polymerase chain reaction (RT-PCR).

RESULTS

KO mice had a threefold increase in renal calcium excretion when compared with WT mice at the baseline. This difference disappeared when calcium load was reduced by overnight fasting. After the CsA treatment, both WT and KO mice had a significant increase of renal calcium excretion (urine Ca/Cr ratio in WT, 0.11 +/- 0.01 to 1.29 +/- 0.17; in KO, 0.39 +/- 0.04 to 1.18 +/- 0.13; both P < 0.01). CsA treatment decreased renal calbindin-D28k mRNA by 61%, but did not affect the expression of ECaC and alpha1G. Baseline serum osteocalcin level of KO mice was significantly lower than that of WT mice. After CsA treatment, both groups had a 50% increase in the serum osteocalcin level, indicating increased bone turnover. When mice were treated with both CsA and APD, the increase in serum osteocalcin level was prevented, and renal calcium excretion was significantly lower than that in mice treated with CsA alone. However, there was still a significant increase in the urine Ca/Cr ratio in WT and KO mice compared with pretreatment levels (urine Ca/Cr in WT, 0.11 +/- 0.01 to 0.76 +/- 0.05, P < 0.01; in KO, 0.39 +/- 0.05 to 0.79 +/- 0.06; P < 0.01).

CONCLUSION

Calbindin-D28k KO mice have diet-dependent hypercalciuria and a lower bone turnover rate. CsA treatment suppresses the expression of calbindin-D28k in mice, but has no effects on ECaC and alpha1G gene expression at the mRNA level. The pathogenesis of CsA-induced hypercalciuria involves both down-regulation of calbindin-D28k with subsequent impaired renal calcium reabsorption and CsA-induced high turnover bone disease. Additionally, our results suggest that mechanism(s) independent of calbindin-D28k within the kidney also may contribute to the CsA-induced calcium leak.

Expression of calcium channels along the differentiation of cultured trophoblast cells from human term placenta

Placental transfer of maternal calcium (Ca2+) is carried out in vivo by the syncytiotrophoblast layer. Although this process is crucial for fetal development, it remains poorly understood. Cytotrophoblasts isolated from human term placenta undergo spontaneous syncytiotrophoblast-like morphological and biochemical differentiation in vitro and are thought to reflect in vivo syncytiotrophoblast. In the present study, we characterized the Ca2+ uptake potential and the expression of several Ca2+ channels by human trophoblasts during differentiation in vitro for up to 6 days. Secretion of hCG (specific differentiation marker) and uptake of Ca2+ by trophoblasts increased gradually as a function of days in culture. Both hCG secretion and Ca2+ uptake were maximal on Day 4 and declined on Days 5-6. Expression of the Ca2+ transporter proteins CaT1 and CaT2 was revealed by reverse transcription-polymerase chain reaction in cytotrophoblasts freshly isolated from human term placenta. In addition, messengers for two L-type Ca2+ channel isoforms (alpha(1C) and alpha(1D)) were also detected. Levels of CaT1, CaT2, and L-type Ca2+ channel mRNA increased gradually during culture, reaching a maximum between Days 2 and 3. In contrast to CaT1 and CaT2 expression that declined thereafter to levels observed on Day 1, L-type channel expression decreased by 50% but remained above the expression level of Day 1. Our results indicate that the pattern of CaT1 and CaT2 expression correlates with the Ca2+ uptake potential along the differentiation of cultured human trophoblasts isolated from term placenta. This correlation provides circumstantial evidence for a role of this family of channels in basal Ca2+ uptake by the syncytiotrophoblast.

Modulation of renal calcium handling by 11 beta-hydroxysteroid dehydrogenase type 2

Reduced concentration of serum ionized calcium and increased urinary calcium excretion have been reported in primary aldosteronism and glucocorticoid-treated patients. A reduced activity of the 11 beta-hydroxysteroid dehydrogenase type 2 (11 beta HSD2) results in overstimulation of the mineralocorticoid receptor by cortisol. Whether inhibition of the 11 beta HSD2 by glycyrrhetinic acid (GA) may increase renal calcium excretion is unknown. Serum and urinary electrolyte and creatinine, serum ionized calcium, urinary calcium excretion, and the steroid metabolites (THF+5 alpha THF)/THE as a parameter of 11 beta HSD2 activity were repeatedly measured in 20 healthy subjects during baseline conditions and during 1 wk of 500 mg/d GA. One week of GA induced a maximal increment of 93% in (THF+5 alpha THF)/THE. Ambulatory BP was significantly higher at day 7 of GA than at baseline (126/77 +/- 10/7 versus 115/73 +/- 8/6 mmHg; P < 0.001 for systolic; P < 0.05 for diastolic). During GA administration, serum ionized calcium decreased from 1.26 +/- 0.05 to 1.18 +/- 0.04 mmol/L (P < 0.0001), and absolute urinary calcium excretion was enhanced from 29.2 +/- 3.6 to 31.9 +/- 3.1 micromol/L GFR (P < 0.01). Fractional calcium excretion increased from 2.4 +/- 0.3 to 2.7 +/- 0.3% (P < 0.01) and was negatively correlated to the fractional sodium excretion during GA (R = -0.35; P < 0.001). Moreover, serum potassium correlated positively with serum ionized calcium (R = 0.66; P < 0.0001). Inhibition of 11 beta HSD2 activity is sufficient to significantly increase the fractional excretion of calcium and decrease serum ionized calcium, suggesting decreased tubular reabsorption of this divalent cation under conditions of renal glucocorticoid/mineralocorticoid excess. The likely site of steroid-regulated renal calcium handling appears to be the distal tubule.

A family of calcium-permeable channels in the kidney: distinct roles in renal calcium handling

PURPOSE OF REVIEW

Calcium is an essential intracellular messenger and a major component of the mineral phase of the skeleton. Calcium is absorbed in the intestine and reabsorbed in the kidney. The underlying transepithelial calcium transport mechanisms play crucial roles in calcium homeostasis. In this review, we present new developments in the area of calcium transport at the apical membrane, the first step in transepithelial calcium transport.

RECENT FINDINGS

Recently, a group of transient receptor potential (TRP)-related calcium-permeable channels has been identified. Several of these channels serve as important epithelial calcium entry mechanisms and possibly also as osmolarity sensors.

SUMMARY

Calcium channels in the kidney play important roles in maintaining total body calcium homeostasis. Their dysfunction may be associated with several human diseases such as hypercaliuric nephrolithiasis, certain forms of osteoporosis, Gitelman's disease and Bartter's syndrome.

Calcium uptake and calcium transporter expression by trophoblast cells from human term placenta

Placental transfer of maternal calcium (Ca(2+)) is a crucial step for fetal development although the biochemical mechanisms responsible for this process are largely unknown. This process is carried out in vivo by the placental syncytiotrophoblast layer. The aim of this study was to define the membrane gates responsible for the syncytiotrophoblast Ca(2+) entry, the first step in transplacental transfer. We have investigated the basal Ca(2+) uptake by primary culture of human term placenta syncytiotrophoblast. Kinetic studies revealed an active extracellular Ca(2+) uptake by cultured human syncytiotrophoblast. We demonstrated by Northern blot the presence of transcript for calcium transporter type 1 (CaT1) in cultured human syncytiotrophoblast and CaT1 expression was further confirmed by reverse transcription polymerase chain reaction (RT-PCR). In addition, the expression of calcium transporter type 2 (CaT2) was revealed by RT-PCR in cultured human syncytiotrophoblast. It has been reported that the activity of this family of Ca(2+) channels is voltage-independent, and is not sensitive to L-type Ca(2+) channels agonist and antagonist. Interestingly, modulation of membrane potential by extracellular high potassium concentration and valinomycin had no effect on the basal Ca(2+) uptake of human syncytiotrophoblast. Moreover, the addition of L-type Ca(2+) channel modulators (Bay K 8644 and nitrendipine) to the incubation medium had also no effect on the basal Ca(2+) uptake, suggesting that the process is mainly voltage-independent and does not involved L-type Ca(2+) channels. On the other hand, we observed that two known blockers of CaT-mediated Ca(2+) transport, namely extracellular magnesium (Mg(2+)) and ruthenium red, dose-dependently inhibited Ca(2+) uptake by cultured human syncytiotrophoblast. Therefore, our results suggest that basal Ca(2+) uptake of human syncytiotrophoblast may be assured by CaT1 and CaT2.

[Fundamental and applied studies on transport and metabolism of electrolytes and glucose--aim to contact with molecular biology]

The authors' research focuses on polyuria, natriuresis, glucosuria, glycemia, and renal calcification in occupational lead poisoning and endemic fluorosis. Changes in electrolyte mobilization and in glucose metabolism and transport following the administration of lead compounds or fluoride were examined to elucidate these mechanisms. The results suggest fundamental approaches to the mechanism of aging and life style diseases. Our results show that: 1) Natriuresis and polyuria in lead poisoning and fluorosis are due to a decrease in renal Na/K-ATPase activity; 2) Renal calcification in fluorosis is due to stimulation of parathyroid function and activation of the renal phosphatidylinositol cascade; 3) Glycemia in fluorosis is due to elevation of renal and hepatic glucose-6-phosphatase activities; 4) Glusosuria in fluorosis is due to decreased renal Na/K-ATPase activity (but fluoride administered directly did not damage the renal Na/glucose cotransporter (SGLT); 5) Renal calcification in fluorosis is due to stimulation of parathyroid function; and 6) The decrease in renal Na/K-ATPase and SGLT activities with aging and hypertension is due to a decrease in phosphorylation activity by protein kinase C (PKC) etc. (decrease in PKC productivity with aging and hypertension).

Magnesium and the parathyroid

The serum levels of parathyroid hormone and magnesium depend on each other in a complex manner. The secretion of parathyroid hormone by the parathyroid is physiologically controlled by the serum calcium level, but magnesium can exert similar effects. While low levels of magnesium stimulate parathyroid hormone secretion, very low serum concentrations induce a paradoxical block. This block leads to clinically relevant hypocalcemia in severely hypomagnesiemic patients. The mechanism of this effect has recently been traced to an activation of the alpha-subunits of heterotrimeric G-proteins. This activation mimicks activation of the calcium sensing receptor and thus causes inhibition of parathyroid hormone secretion. In addition to the effects of magnesium on parathyroid hormone secretion, parathyroid hormone in turn regulates magnesium homeostasis by modulating renal magnesium reabsorption. The distal convoluted tubule is of crucial importance for parathyroid hormone-regulated magnesium homeostasis.

Deletion of deoxyribonucleic acid binding domain of the vitamin D receptor abrogates genomic and nongenomic functions of vitamin D

The vitamin D hormone 1,25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)], the biologically active form of vitamin D, is essential for an intact mineral metabolism. Using gene targeting, we sought to generate vitamin D receptor (VDR) null mutant mice carrying the reporter gene lacZ driven by the endogenous VDR promoter. Here we show that our gene-targeted mutant mice express a VDR with an intact hormone binding domain, but lacking the first zinc finger necessary for DNA binding. Expression of the lacZ reporter gene was widely distributed during embryogenesis and postnatally. Strong lacZ expression was found in bones, cartilage, intestine, kidney, skin, brain, heart, and parathyroid glands. Homozygous mice are a phenocopy of mice totally lacking the VDR protein and showed growth retardation, rickets, secondary hyperparathyroidism, and alopecia. Feeding of a diet high in calcium, phosphorus, and lactose normalized blood calcium and serum PTH levels, but revealed a profound renal calcium leak in normocalcemic homozygous mutants. When mice were treated with pharmacological doses of vitamin D metabolites, responses in skin, bone, intestine, parathyroid glands, and kidney were absent in homozygous mice, indicating that the mutant receptor is nonfunctioning and that vitamin D signaling pathways other than those mediated through the classical nuclear receptor are of minor physiological importance. Furthermore, rapid, nongenomic responses to 1,25-(OH)(2)D(3) in osteoblasts were abrogated in homozygous mice, supporting the conclusion that the classical VDR mediates the nongenomic actions of 1,25-(OH)(2)D(3).

Polarized calcium and calmodulin signaling in secretory epithelia

This review examines polarized calcium and calmodulin signaling in exocrine epithelial cells. The calcium ion is a simple, evolutionarily ancient, and universal second messenger. In exocrine epithelial cells, it regulates essential functions such as exocytosis, fluid secretion, and gene expression. Exocrine cells are structurally polarized, with the apical region usually dedicated to secretion. Recent advances in technology, in particular the development of videoimaging and confocal microscopy, have led to the discovery of polarized, subcellular calcium signals in these cell types. The properties of a rich variety of local and global calcium signals have now been described in secretory epithelial cells. Secretagogues stimulate apical-to-basal waves of calcium in many exocrine cell types, but there are some interesting exceptions to this rule. The shapes of intracellular calcium signals are determined by the distribution of calcium-releasing channels and mechanisms that limit calcium elevation. Polarized distribution of calcium-handling mechanisms also leads to transcellular calcium transport in exocrine epithelial cells. This transport can deliver considerable amounts of calcium into secreted fluids. Multicellular polarized calcium signals can coordinate the activity of many individual cells in epithelial secretory tissue. Certain particularly sensitive cells serve as pacemakers for initiation of intercellular calcium waves. Many calcium signaling pathways involve activation of calmodulin. This ubiquitous protein regulates secretion in exocrine cells and also activates interesting feedback interactions with calcium channels and transporters. Very recently it became possible to directly study polarized calcium-calmodulin reactions and to visualize the process of hormone-induced redistribution of calmodulin in live cells. The structural and functional polarity of secretory epithelia alongside the polarity of its calcium and calmodulin signaling present an interesting lesson in tissue organization.

Current understanding of mammalian TRP homologues

Calcium influx into the cell from the extracellular medium is crucial for important processes including muscle contraction, secretion and gene expression. This calcium influx is mainly mediated through calcium influx channels, which on the basis of their activation mechanism can be subdivided in voltage-gated calcium channels, which have already been thoroughly characterized and non-voltage-gated calcium permeable channels. This latter group includes ion channels activated by binding of extra and intracellular messengers, mechanical stress or depletion of intracellular calcium stores. Currently little molecular data is available concerning this class of calcium influx channels. However, recent studies have indicated that members of the transient receptor potential (TRP) family of ion channels can function as calcium influx channels both in excitable and non-excitable tissues. On the basis of structural information the TRP family is subdivided in three main subfamilies: the TRPC (canonical) group, the TRPV (vanilloid) group and the TRPM (melastatin) group. The cloning and characterization of members of this cation channel family has exploded during recent years, leading to a plethora of data concerning TRPs in a variety of tissues and species, including mammals, insects and yeast. This review summarizes the currently available information concerning members of the TRP family expressed in mammalian tissues.

Vanilloid and TRP channels: a family of lipid-gated cation channels

The emergence of the TRP (C) and vanilloid (TRPV) receptor family of Ca(2+) permeable channels has started to provide molecular focus to a linked group of ion channels whose common feature is activation primarily by intracellular ligands. These channels have a central role in Ca(2+) homeostasis in virtually all cells and in particular those that lack voltage-gated Ca(2+) channels. We will discuss recent work that is more precisely defining both molecular form and physiological function of this important group of Ca(2+) permeable channels with particular focus on the intracellular ligands that gate and modulate channel activity.

Molecular mechanism of active Ca2+ reabsorption in the distal nephron

The identification of the epithelial Ca(2+) channel (ECaC) complements the group of Ca(2+) transport proteins including calbindin-D28K, Na(+)/Ca(2+) exchanger and plasma membrane Ca(2+)-ATPase, which are co-expressed in 1,25(OH)2D3- responsive nephron segments. ECaC constitutes the rate-limiting apical entry step in the process of active transcellular Ca(2+) transport and belongs to a superfamily of Ca(2+) channels that includes the vanilloid receptor and transient receptor potential channels. This new Ca(2+) channel consists of six transmembrane-spanning domains, including a pore-forming hydrophobic stretch between domain 5 and 6. The C- and N-terminal tails contain several conserved regulatory sites, implying that the channel function is modulated by regulatory proteins. The distinctive functional properties of ECaC include a constitutively activated Ca(2+) permeability, a high selectivity for Ca(2+), hyperpolarization-stimulated and Ca(2+)-dependent feedback regulation of channel activity, and 1,25(OH)2D3-induced gene activation. This review covers the distinctive properties of this new highly Ca(2+)-selective channel and highlights the implications for active transcellular Ca(2+) reabsorption in health and disease.

Renal genetic disorders related to K+ and Mg2+

The recent knowledge of the renal epithelial transport systems has exploded with the identification, cloning, and characterization of a large number of membrane transport proteins. The fundamental aspects of these transporters are beginning to emerge at the molecular level and are summarized in the accompanying contributions in this volume of the Annual Review of Physiology. The aim of my review is to integrate this body of knowledge with the understanding of the clinical disorders of human mineral homeostasis that accompany gain, loss, or dysregulation of function of these transport systems. The specific focus is on the best defined human clinical syndromes in which there are derangements in K(+) and Mg(2+) homeostasis.

Role of aspartate residues in Ca(2+) affinity and permeation of the distal ECaC1

The Ca(2+) affinity and permeation of the epithelial Ca(2+) channel (ECaC1) were investigated after expression in Xenopus oocytes. ECaC1 displayed anomalous mole-fraction effects. Extracellular Ca(2+) and Mg(2+) reversibly inhibited ECaC1 whole cell Li(+) currents: IC(50) = 2.2 +/- 0.4 microM (n = 9) and 235 +/- 35 microM (n = 10), respectively. These values compare well with the Ca(2+) affinity of the L-type voltage-gated Ca(2+) (Ca(V)1.2) channel measured under the same conditions, suggesting that high-affinity Ca(2+) binding is a well-conserved feature of epithelial and voltage-gated Ca(2+) channels. Neutralization of D550 and E535 in the pore region had no significant effect on Ca(2+) and Mg(2+) affinities. In contrast, neutralization of D542 significantly decreased Ca(2+) affinity (IC(50) = 1.1 +/- 0.2 mM, n = 6) and Mg(2+) affinity (IC(50) > 25 +/- 3 mM, n = 4). Despite a 1,000-fold decrease in Ca(2+) affinity in D542N, Ca(2+) permeation properties and the Ca(2+)-to-Ba(2+) conductance ratio remained comparable to values for wild-type ECaC1. Together, our observations suggest that D542 plays a critical role in Ca(2+) affinity but not in Ca(2+) permeation in ECaC1.

The diversity in the vanilloid (TRPV) receptor family of ion channels

Following cloning of the vanilloid receptor 1 (VR1) at least four other related proteins have been identified. Together, these form a distinct subgroup of the transient receptor potential (TRP) family of ion channels. Members of the vanilloid receptor family (TRPV) are activated by a diverse range of stimuli, including heat, protons, lipids, phorbols, phosphorylation, changes in extracellular osmolarity and/or pressure, and depletion of intracellular Ca2+ stores. However, VR1 remains the only channel activated by vanilloids such as capsaicin. These channels are excellent molecular candidates to fulfil a range of sensory and/or cellular roles that are well characterized physiologically. Furthermore, as novel pharmacological targets, the vanilloid receptors have potential for the development of many future disease treatments.

Tubular and cellular localization of the cardiac L-type calcium channel in rat kidney

BACKGROUND

The mRNAs of several types of calcium channels have been identified in intact rat kidney, and L-type calcium channels cause changes in intracellular calcium in primary cultures of distal tubule cells. The aim of this study was to evaluate the tubular and cellular distribution of the alpha1C subunit of the L-type calcium channel in intact kidney.

METHODS

RT-PCR and Northern blot analysis were used to assess the regional abundance of the mRNA of this channel. Immunocytochemistry combined with confocal microscopy and surface biotinylation were applied to determine the tubular and cellular localization of the protein.

RESULTS

Northern blot and RT-PCR analysis indicated that the mRNA of the alpha1C subunit of the cardiac L-type calcium channel was present in whole rat kidney, kidney tubules and kidney cell lines. Western blot of lysates from whole kidney, kidney tubules or cell lines revealed bands of approximately 190 kD for the alpha1C subunit and approximately 60 kD for the beta3 subunit. Confocal immunohistochemistry indicated that the alpha1C subunit of this channel was co-expressed in cells of the distal tubule that express calbindin-D28K, but not in intercalated cells. The alpha1C subunit was also highly expressed in both outer and inner medullary collecting ducts. Serial confocal microscopic images or surface biotinylation experiments determined that the channel was predominantly on the basolateral membrane but had some distribution on the apical membrane.

CONCLUSIONS

The distribution and cellular localization of the alpha1C subunit of cardiac L-type calcium channel suggest it is probably involved in intracellular and membrane calcium signaling.

TRP channel proteins and signal transduction

TRP channel proteins constitute a large and diverse family of proteins that are expressed in many tissues and cell types. This family was designated TRP because of a spontaneously occurring Drosophila mutant lacking TRP that responded to a continuous light with a transient receptor potential (hence TRP). In addition to responses to light, TRPs mediate responses to nerve growth factor, pheromones, olfaction, mechanical, chemical, temperature, pH, osmolarity, vasorelaxation of blood vessels, and metabolic stress. Furthermore, mutations in several members of TRP-related channel proteins are responsible for several diseases, such as several tumors and neurodegenerative disorders. TRP-related channel proteins are found in a variety of organisms, tissues, and cell types, including nonexcitable, smooth muscle, and neuronal cells. The large functional diversity of TRPs is also reflected in their diverse permeability to ions, although, in general, they are classified as nonselective cationic channels. The molecular domains that are conserved in all members of the TRP family constitute parts of the transmembrane domains and in most members also the ankyrin-like repeats at the NH2 terminal of the protein and a "TRP domain" at the COOH terminal, which is a highly conserved 25-amino acid stretch with still unknown function. All of the above features suggest that members of the TRP family are "special assignment" channels, which are recruited to diverse signaling pathways. The channels' roles and characteristics such as gating mechanism, regulation, and permeability are determined by evolution according to the specific functional requirements.

Gene structure and regulation of the murine epithelial calcium channels ECaC1 and 2

The recently discovered epithelial calcium channels ECaC1 and ECaC2 are thought to play an important role in active calcium absorption in the intestine and kidney. Vitamin D-responsive elements (VDRE) were detected in the promoter sequence of human ECaC1 and regulation of ECaC by the steroid hormone 1,25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)) has been postulated. In this study we describe the structure of two murine ECaCs genes, each consisting of 15 exons localized on chromosome 6. Murine ECaC2 expression was found in many target tissues of 1,25-(OH)(2)D(3), including skin and osteoblastic cells, while ECaC1 expression is confined to the kidney. By screening the murine promoter sequences, we detected a putative VDRE in ECaC1 and an estrogen response element in ECaC2. However, experiments in mice with a mutant, nonfunctioning vitamin D receptor showed that expression of ECaC1 in the kidney and of ECaC2 in duodenum is regulated by calcium levels, but not by 1,25-(OH)(2)D(3). Also, estrogen-deficient ovariectomized (OVX) mice and OVX mice supplemented with estradiol showed unchanged duodenal ECaC2 expression compared with control mice. We conclude that ECaC expression in the kidney and the intestine is regulated by extracellular calcium but not by vitamin D or estrogen in vivo in mice.

Function and expression of the epithelial Ca(2+) channel family: comparison of mammalian ECaC1 and 2

1. The epithelial Ca(2+) channel (ECaC) family represents a unique group of Ca(2+)-selective channels that share limited homology to the ligand-gated capsaicin receptors, the osmolarity-sensitive channel OTRPC4, as well as the transient receptor potential family. Southern blot analysis demonstrated that this family is restricted to two members, ECaC1 and ECaC2 (also named CaT1). 2. RT-PCR analysis demonstrated that the two channels are co-expressed in calbindin-D-containing epithelia, including small intestine, pancreas and placenta, whereas kidney and brain only express ECaC1 and stomach solely ECaC2. 3. From an electrophysiological point of view, ECaC1 and ECaC2 are highly similar channels. Differences concern divalent cation permeability, the kinetics of Ca(2+)-dependent inactivation and recovery from inactivation. 4. Ruthenium red is a potent blocker of ECaC activity. Interestingly, ECaC2 has a 100-fold lower affinity for ruthenium red (IC(50) 9 +/- 1 microM) than ECaC1 (IC(50) 121 +/- 13 nM). 5. ECaCs are modulated by intracellular Mg(2+) and ATP. ECaC1 and ECaC2 activity rapidly decay in the absence of intracellular ATP. This effect is further accelerated at higher intracellular Mg(2+) concentrations. 6. In conclusion, ECaC1 and ECaC2 are homologous channels, with an almost identical pore region. They can be discriminated by their sensitivity for ruthenium red and show differences in Ca(2+)-dependent regulation.

Claudin-2 is selectively expressed in proximal nephron in mouse kidney

The proximal nephron possesses a leaky epithelium with unique paracellular permeability properties that underlie its high rate of passive NaCl and water reabsorption, but the molecular basis is unknown. The claudins are a large family of transmembrane proteins that are part of the tight junction complex and likely form structural components of a paracellular pore. To localize claudin-2 in the mouse kidney, we performed in situ hybridization using an isoform-specific riboprobe and immunohistochemistry using a polyclonal antibody directed against a COOH-terminal peptide. Claudin-2 mRNA and protein were found throughout the proximal tubule and in the contiguous early segment of the thin descending limb of long-looped nephrons. The level of expression demonstrated an axial increase from proximal to distal segments. In confocal images, the subcellular localization of claudin-2 protein coincided with that of the tight junction protein ZO-1. Our findings suggest that claudin-2 is a component of the paracellular pathway of the most proximal segments of the nephron and that it may be responsible for their uniquely leaky permeability properties.

Claudin-2 is selectively expressed in proximal nephron in mouse kidney

First published August 15, 2001; 10.1152/ajprenal.00021.2001.—The proximal nephron possesses a leaky epithelium with unique paracellular permeability properties that underlie its high rate of passive NaCl and water reabsorption, but the molecular basis is unknown. The claudins are a large family of transmembrane proteins that are part of the tight junction complex and likely form structural components of a paracellular pore. To localize claudin-2 in the mouse kidney, we performed in situ hybridization using an isoform-specific riboprobe and immunohistochemistry using a polyclonal antibody directed against a COOH-terminal peptide. Claudin-2 mRNA and protein were found throughout the proximal tubule and in the contiguous early segment of the thin descending limb of long-looped nephrons. The level of expression demonstrated an axial increase from proximal to distal segments. In confocal images, the subcellular localization of claudin-2 protein coincided with that of the tight junction protein ZO-1. Our findings suggest that claudin-2 is a component of the paracellular pathway of the most proximal segments of the nephron and that it may be responsible for their uniquely leaky permeability properties.

Pharmacological modulation of monovalent cation currents through the epithelial Ca2+ channel ECaC1

1. The recent identification of the epithelial Ca(2+) channel, ECaC1, represents a major step forward in our knowledge of renal Ca(2+) handling. ECaC1 constitutes the rate-limiting apical Ca(2+) entry mechanism of active, transcellular Ca(2+) reabsorption. This unique highly selective Ca(2+) channel shares a low but significant homology with transient receptor potential (TRP) channels and vanilloid receptors (VR). 2. We have studied the pharmacological modulation of currents through ECaC1 heterologously expressed in HEK 293 cells. Monovalent cation currents were measured by use of the whole cell patch clamp technique in cells dialysed with 10 mM BAPTA or 10 mM EGTA to prevent the fast Ca(2+) dependent inactivation of ECaC1. 3. Several modulators were tested, including inorganic cations, putative store-operated Ca(2+) entry (SOC) blockers, the vanilloid receptor (VR-1) blocker capsazepine, protein tyrosine kinase blockers, calmodulin antagonists and ruthenium red. 4. Ruthenium red and econazole appeared to be the most effective inhibitors of currents through ECaC1, with IC(50) values of 111 nM and 1.3 microM, respectively, whereas the selective SOC inhibitor, SKF96365, was nearly ineffective. 5. The divalent cation current block profile for ECaC1 is Pb(2+)=Cu(2+) >Zn(2+) >Co(2+) >Fe(2+) with IC(50) values between 1 and approximately 10 microM. 6. In conclusion, ECaC activity is effectively inhibited by various compounds including ruthenium red, antimycotic drugs and divalent cations, which might be useful tools for pharmacological manipulation and several disorders related to Ca(2+) homeostasis could benefit from such developments.

Characteristics of calcium uptake by BeWo cells, a human trophoblast cell line

Placental transfer of maternal calcium (Ca(2+)) is a crucial process for fetal development although the biochemical mechanisms responsible for this transfer are largely unknown. We have investigated the characteristics of Ca(2+)uptake by the human placental trophoblast cell line BeWo. The kinetics studies revealed an active extracellular Ca(2+)uptake by BeWo cells, which was rapid in the first 2 min (initial velocity (V(i)) of 4.17+/-0.25 nmol/mg/min), and showed a subsequent plateau. Uptake experiments performed at V(i)with increasing concentrations of Ca(2+)resulted in a typical saturation curve (K(m)of 0.54+/-0.07 m m and V(max)of 7.07+/-0.28 nmol/mg protein/min). Lowering the pH of the incubation medium from 7.4 to 5.5 led to Ca(2+)uptake inhibition of 40-50 per cent. The presence of voltage-sensitive (l -type) Ca(2+)channels in BeWo cells was demonstrated by Western blot. Therefore, the implication of such channels in basal Ca(2+)uptake of BeWo cells was investigated. Cell depolarization with extracellular high potassium concentration (40 m m), and hyperpolarization with extracellular high chloride concentration (60 m m) or with valinomycin (10 microm) did not influence the basal Ca(2+)uptake of BeWo cells. The L-type Ca(2+)channel modulators (Bay K 8644 and Nitrendipine) had no effect on the Ca(2+)uptake. An antagonist of receptor-mediated, store-operated and voltage-gated Ca(2+)channels (SKF-96365) also did not modulate the Ca(2+)uptake of BeWo cells. Therefore, our results indicate that the basal Ca(2+)uptake of BeWo cells is inhibited by lowering pH of the incubation medium, is voltage independent, and is not influenced by l -type Ca(2+)channel and capacitative Ca(2+)conductance modulators.

Endoplasmic reticulum Ca2+-ATPase pump is up-regulated in calcium-transporting dental enamel cells: a non-housekeeping role for SERCA2b

Dental enamel-forming cells face a major challenge to avoid the cytotoxic effects of excess calcium. We have characterized sarcoplasmic/endoplasmic reticulum calcium-ATPase pumps (SERCA) in rat enamel cells to address the proposal that non-mitochondrial calcium stores play a dominant role in transcellular calcium transport. A single major isoform, SERCA2b, was detected during the protein-secretory and calcium-transport stages of enamel formation using reverse-transcriptase PCR, cDNA cloning, Northern analysis and immunoblotting. Most importantly, SERCA2b exhibited a specific 3-fold up-regulation to high expression levels during calcium transport, as determined by quantitative immunoblotting and ATPase assays. Sensitivity of the calcium-dependent ATPase to thapsigargin and three other SERCA inhibitors was characterized. These findings indicate that enamel cells are well-equipped to sequester calcium in endoplasmic reticulum stores and so protect against calcium toxicity, associate SERCA with transcellular calcium transport for the first time, and establish SERCA2b as a molecular and pharmacological target for future investigations of calcium transcytosis. The observed physiological regulation in enamel cells contradicts the widespread perception that SERCA2b is restricted to general housekeeping duties.

Structural conservation of the genes encoding CaT1, CaT2, and related cation channels

We report here the genomic structures of the genes encoding human calcium transport proteins CaT1 and CaT2, which belong to a recently identified class of highly selective calcium entry channels. The mRNA for CaT1 was expressed more abundantly than that for CaT2 in three major tissues involved in transcellular calcium transport, namely intestine, kidney, and placenta, as determined by quantitative PCR. The genes encoding CaT1 and CaT2, ECAC2 and ECAC1, respectively, are completely conserved in terms of exon size in the coding regions. They also share similar intron-exon structures with the genes encoding the closely related, nonselective cation channels VR1, VRL-1, OTRPC4 (also known as VR-OAC, Trp12, and VRL-2), and a hypothetical protein, VRL-3. We conclude that ECAC2 and ECAC1, which encode calcium selective channels, share a common ancestral gene with the genes encoding the related nonselective cation channels.

Recent developments in non-excitable cell calcium entry

Influx of calcium into cells following stimulation of cell surface receptors is a key process controlling cellular activity. However, despite intensive research, there is still no consensus on precisely how calcium entry is controlled in electrically no n-excitable cells. In particular, the regulation of depletion-activated or 'capacitative' calcium entry continues to be a focus of debate. Work published in the last 2 years has lent new impetus to the so-called 'conformational coupling' theory, although evidence for the existence of soluble messengers between the ER and the plasma membrane also continues to appear. In addition, there remains disagreement on whether intra-store [Ca(2+)] has to fall below a threshold before Ca(2+)entry is activated. A further major question is the identity of the putative depletion-operated Ca(2+)channel or channels. Here discussion has largely focussed on whether homologue(s) of the Drosophila TRP ('Transient Receptor Potential') protein is/are the elusive channel, or at least a part of it. Finally, it remains possible that Ca(2+)entry mechanisms other than depletion-activated channels may be important in agonist-evoked Ca(2+)influx. This commentary summarizes recent developments in the field, and highlights both current debates and critical unsolved questions.

Modulation of the epithelial calcium channel, ECaC, by intracellular Ca2+

We have studied the modulation by intracellular Ca2+ of the epithelial Ca2+ channel, ECaC, heterologously expressed in HEK 293 cells. Whole-cell and inside-out patch clamp current recordings were combined with FuraII-Ca2+ measurements:1. Currents through ECaC were dramatically inhibited if Ca2+ was the charge carrier. This inhibition was dependent on the extracellular Ca2+ concentration and occurred also in cells buffered intracellularly with 10 mM BAPTA.2. Application of 30 mM [Ca(2)]e induced in non-Ca2+] buffered HEK 293 cells at -80 m V an increase in intracellular Ca2+([Ca2]i) with a maximum rate of rise of 241 +/-15nM/s (n= 18 cells) and a peak value of 891 +/- 106 nM. The peak of the concomitant current with a density of 12.3 +/- 2.6 pA/pF was closely correlated with the peak of the first-time derivative of the Ca2+ transient, as expected if the Ca2+ transient is due to influx of Ca2+. Consequently, no Ca2+] signal was observed in cells transfected with the Ca2+ impermeable ECaC mutant, D542A, in which an aspartate in the pore region was neutralized.3. Increasing [Ca2+]i by dialyzing the cell with pipette solutions containing various Ca2+] concentrations, all buffered with 10 mM BAPTA, inhibited currents through ECaC carried by either Na+ or Ca2+] ions. Half maximal inhibition of Ca(2+)currents in the absence of monovalent cations occurred at 67 nM (n between 6 and 8), whereas Na+ currents in the absence of Ca2+] and Mg2+ were inhibited with an IC50 of 89 nM (n between 6 and 10). Currents through ECaC in the presence of 1 mM Ca2+ and Na+, which are mainly carried by Ca2+, are inhibited by [Ca2]i with an IC50of 82 nM (n between 6 and 8). Monovalent cation currents through the Ca2+impermeable D542A ECaC mutant were also inhibited by an elevation of [Ca2]i (IC50 = 123 nM, n between 7 and 18). 4. The sensitivity of ECaC currents in inside-out patches for [Ca2]i was slightly shifted to higher concentrations as compared with whole cell measurements. Half-maximal inhibition occurred at 169 nM if Na+ was the charge carrier (n between 4 and 11) and 228 nM at 1 mM [Ca2]e (n between 4 and 8).5. Recovery from inhibition upon washout of extracellular Ca2+ (whole-cell configuration) or removal of Ca2+ from the inner side of the channel (inside-out patches) was slow in both conditions. Half-maximal recovery was reached after 96 +/- 34 s (n= 15) in whole-cell mode and after 135 +/- 23 s (n = 17) in inside-out patches.6. We conclude that influx of Ca2+ through ECaC and [Ca2]i induce feedback inhibition of ECaC currents, which is controlled by the concentration of Ca2+ in a micro domain near the inner mouth of the channel. Slow recovery seems to depend on dissociation of Ca( 2+ from an internal Ca2+ binding site at ECaC.

The TRP ion channel family

Mammalian homologues of the Drosophila transient receptor potential (TRP) channel gene encode a family of at least 20 ion channel proteins. They are widely distributed in mammalian tissues, but their specific physiological functions are largely unknown. A common theme that links the TRP channels is their activation or modulation by phosphatidylinositol signal transduction pathways. The channel subunits have six transmembrane domains that most probably assemble into tetramers to form non-selective cationic channels, which allow for the influx of calcium ions into cells. Three subgroups comprise the TRP channel family; the best understood of these mediates responses to painful stimuli. Other proposed functions include repletion of intracellular calcium stores, receptor-mediated excitation and modulation of the cell cycle.