Toward a comprehensive molecular model of active calcium reabsorption

Contributions of SGK3 to transporter-related diseases

Serum- and glucocorticoid-induced kinase 3 (SGK3), which is ubiquitously expressed in mammals, is regulated by estrogens and androgens. SGK3 is activated by insulin and growth factors through signaling pathways involving phosphatidylinositol-3-kinase (PI3K), 3-phosphoinositide-dependent kinase-1 (PDK-1), and mammalian target of rapamycin complex 2 (mTORC2). Activated SGK3 can activate ion channels (TRPV5/6, SOC, Kv1.3, Kv1.5, Kv7.1, BKCa, Kir2.1, Kir2.2, ENaC, Nav1.5, ClC-2, and ClC Ka), carriers and receptors (Npt2a, Npt2b, NHE3, GluR1, GluR6, SN1, EAAT1, EAAT2, EAAT4, EAAT5, SGLT1, SLC1A5, SLC6A19, SLC6A8, and NaDC1), and Na+/K+-ATPase, promoting the transportation of calcium, phosphorus, sodium, glucose, and neutral amino acids in the kidney and intestine, the absorption of potassium and neutral amino acids in the renal tubules, the transportation of glutamate and glutamine in the nervous system, and the transportation of creatine. SGK3-sensitive transporters contribute to a variety of physiological and pathophysiological processes, such as maintaining calcium and phosphorus homeostasis, hydro-salinity balance and acid-base balance, cell proliferation, muscle action potential, cardiac and neural electrophysiological disturbances, bone density, intestinal nutrition absorption, immune function, and multiple substance metabolism. These processes are related to kidney stones, hypophosphorous rickets, multiple syndromes, arrhythmia, hypertension, heart failure, epilepsy, Alzheimer's disease, amyotrophic lateral sclerosis, glaucoma, ataxia idiopathic deafness, and other diseases.

The Role of Vitamin D in Small Animal Bone Metabolism

Dogs and cats have differences in vitamin D metabolism compared to other mammalian species, as they are unable to perform vitamin D cutaneous synthesis through sun exposure. Therefore, they are dependent on the dietary intake of this nutrient. The classic functions of vitamin D are to stimulate intestinal calcium and phosphate absorption, renal calcium and phosphate reabsorption and regulate bone mineral metabolism. Thus, it is an important nutrient for calcium and phosphorus homeostasis. This review highlights the evidence of the direct and indirect actions of vitamin D on bone mineral metabolism, the consequences of nutritional imbalances of this nutrient in small animals, as well as differences in vitamin D metabolism between different size dogs.

Calcium Extrusion Pump PMCA4: A New Player in Renal Calcium Handling?

Calcium (Ca²⁺) is vital for multiple processes in the body, and maintenance of the electrolyte concentration is required for everyday physiological function. In the kidney, and more specifically, in the late distal convoluted tubule and connecting tubule, the fine-tuning of Ca²⁺ reabsorption from the pro-urine takes place. Here, Ca²⁺ enters the epithelial cell via the transient receptor potential vanilloid receptor type 5 (TRPV5) channel, diffuses to the basolateral side bound to calbindin-D_28k and is extruded to the blood compartment via the Na⁺/Ca²⁺ exchanger 1 (NCX1) and the plasma membrane Ca²⁺ ATPase (PMCA). Traditionally, PMCA1 was considered to be the primary Ca²⁺ pump in this process. However, in recent studies TRPV5-expressing tubules were shown to highly express PMCA4. Therefore, PMCA4 may have a predominant role in renal Ca²⁺ handling. This study aimed to elucidate the role of PMCA4 in Ca²⁺ homeostasis by characterizing the Ca²⁺ balance, and renal and duodenal Ca²⁺-related gene expression in PMCA4 knockout mice. The daily water intake of PMCA4 knockout mice was significantly lower compared to wild type littermates. There was no significant difference in serum Ca²⁺ level or urinary Ca²⁺ excretion between groups. In addition, renal and duodenal mRNA expression levels of Ca²⁺-related genes, including TRPV5, TRPV6, calbindin-D_28k, calbindin-D_9k, NCX1 and PMCA1 were similar in wild type and knockout mice. Serum FGF23 levels were significantly increased in PMCA4 knockout mice. In conclusion, PMCA4 has no discernible role in normal renal Ca²⁺ handling as no urinary Ca²⁺ wasting was observed. Further investigation of the exact role of PMCA4 in the distal convoluted tubule and connecting tubule is required.

Successful management of repetitive urinary obstruction and anuria caused by double j stent calculi formation after renal transplantation

This report firstly describes an extremely rare case of repetitive double J stent calculi formation after renal transplantation caused by the antihyperparathyroidism (HPT) drug calcitriol. In 2012, a woman initially presented to our hospital for anuria with lower abdominal pain. She was diagnosed with allograft hydronephrosis and double J stents obstruction by calculi formation after transplantation and treated with triplicate stents replacements in another hospital without clinical manifestations improvements. Through detailed exploration of medical history, we conclude that the abnormal calculi formation is due to the calcitriol (1,25-dihydroxyvitamin D3) administration, a drug which can increase renal tubular reabsorption of calcium for treating posttransplant HPT bone disease. After discontinuing calcitriol, the patient was stone-free and had a good recovery without severe complications during the 9-month follow-up. Our novel findings may provide an important clue and approach to managing formidable repetitive double J stent calculi formation in the clinical trial.

Calcitriol, calcidiol, parathyroid hormone, and fibroblast growth factor-23 interactions in chronic kidney disease

Objective

To review the inter-relationships between calcium, phosphorus, parathyroid hormone (PTH), parent and activated vitamin D metabolites (vitamin D, 25(OH)-vitamin D, 1,25(OH)₂-vitamin D, 24,25(OH)₂-vitamin D), and fibroblast growth factor-23 (FGF-23) during chronic kidney disease (CKD) in dogs and cats.

Data Sources

Human and veterinary literature.

Human Data Synthesis

Beneficial effects of calcitriol treatment during CKD have traditionally been attributed to regulation of PTH but new perspectives emphasize direct renoprotective actions independent of PTH and calcium. It is now apparent that calcitriol exerts an important effect on renal tubular reclamation of filtered 25(OH)-vitamin D, which may be important in maintaining adequate circulating 25(OH)-vitamin D. This in turn may be vital for important pleiotropic actions in peripheral tissues through autocrine/paracrine mechanisms that impact the health of those local tissues.

Veterinary Data Synthesis

Limited information is available reporting the benefit of calcitriol treatment in dogs and cats with CKD.

Conclusions

A survival benefit has been shown for dogs with CKD treated with calcitriol compared to placebo. The concentrations of circulating 25(OH)-vitamin D have recently been shown to be low in people and dogs with CKD and are related to survival in people with CKD. Combination therapy for people with CKD using both parental and activated vitamin D compounds is common in human nephrology and there is a developing emphasis using combination treatment with activated vitamin D and renin-angiotensin-aldosterone-system (RAAS) inhibitors.

The epithelial calcium channel TRPV5 is regulated differentially by klotho and sialidase

The transient receptor potential vanilloid type 5 (TRPV5) Ca(2+) channel facilitates transcellular Ca(2+) transport in the distal convoluted tubule (DCT) of the kidney. The channel is glycosylated with a complex type N-glycan and it has been postulated that hydrolysis of the terminal sialic acid(s) stimulate TRPV5 activity. The present study delineates the role of the N-glycan in TRPV5 activity using biochemical assays in Human Embryonic Kidney 293 cells expressing TRPV5, isoelectric focusing and total internal reflection fluorescent microscopy. The anti-aging hormone klotho and other glycosidases stimulate TRPV5-dependent Ca(2+) uptake. Klotho was found to increase the plasma membrane stability of TRPV5, via the TRPV5 N-glycan. Sialidase mimicked this stimulatory action. However, this effect was independent of the N-glycosylation state of TRPV5, since the N-glycosylation mutant (TRPV5(N358Q)) was activated to the same extent. We showed that the increased TRPV5 activity after sialidase treatment is caused by inhibition of lipid raft-mediated internalization. In addition, sialidase modified the N-glycan of transferrin, a model glycoprotein, differently from klotho. Previous studies showed that after klotho treatment, galectin-1 binds the TRPV5 N-glycan and thereby increases TRPV5 activity. However, galectin-3, but not galectin-1, was expressed in the DCT. Furthermore, an increase in TRPV5-mediated Ca(2+) uptake was detected after galectin-3 treatment. In conclusion, two distinct TRPV5 stimulatory mechanisms were demonstrated; a klotho-mediated effect that is dependent on the N-glycan of TRPV5 and a sialidase-mediated stimulation that is lipid raft-dependent and independent of the N-glycan of TRPV5.

Expression and regulation of sodium/calcium exchangers, NCX and NCKX, in reproductive tissues: do they play a critical role in calcium transport for reproduction and development?

Plasma membrane sodium/calcium (Na(+)/Ca(2+)) exchangers are an important component of intracellular calcium [Ca(2+)](i) homeostasis and electrical conduction. Na(+)/Ca(2+) exchangers, NCX and NCKX, play a critical role in the transport of one [Ca(2+)](i) and potassium ion across the cell membrane in exchange for four extracellular sodium ions [Na(+)](e). Mammalian plasma membrane Na(+)/Ca(2+) exchange proteins are divided into two families: one in which Ca(2+) flux is dependent only on sodium (NCX1-3) and another in which Ca(2+) flux is also dependent on potassium (NCKX1-4). Both molecules are capable of forward- and reverse-mode exchange. In cells and tissues, Na(+)/Ca(2+) (and K(+)) gradients localize to the cell membrane; thus, the exchangers transport ions across a membrane potential. Uterine NCKX3 has been shown to be involved in the regulation of endometrial receptivity by [Ca(2+)](i). In the uterus and placenta, NCKX3 expression is regulated by the sex steroid hormone estrogen (E2) and hypoxia stress, respectively. In this chapter, we described the expression and regulation of these proteins for reproductive functions in various tissues including uterus, placenta, and kidney of humans and rodents. Evidence to date suggests that NCKX3 and NCX1 may be regulated in a tissue-specific manner. In addition, we focused on the molecular mechanism involved in the regulation of NCKX3 and NCX1 in mammals, based upon our recent results and those of others.

Effects of a high-sodium diet on renal tubule Ca2+ transporter and claudin expression in Wistar-Kyoto rats

Background

Urinary Ca²⁺ excretion increases with dietary NaCl. NaCl-induced calciuria may be associated with hypertension, urinary stone formation and osteoporosis, but its mechanism and long-term effects are not fully understood. This study examined alterations in the expressions of renal Ca²⁺ transporters, channels and claudins upon salt loading to better understand the mechanism of salt-induced urinary Ca²⁺ loss.

Methods

Eight-week old Wistar-Kyoto rats were fed either 0.3% or 8% NaCl diet for 8 weeks. Renal cortical expressions of Na⁺/Ca²⁺ exchanger 1 (NCX1), Ca²⁺ pump (PCMA1b), Ca²⁺ channel (TRPV5), calbindin-D_28k, and claudins (CLDN-2, -7, -8, -16 and −19) were analyzed by quantitative PCR, western blot and/or immunohistochemistry.

Results

Fractional excretion of Ca²⁺ increased 6.0 fold with high-salt diet. Renal cortical claudin-2 protein decreased by approximately 20% with decreased immunological staining on tissue sections. Claudin-16 and −19 expressions were not altered. Renal cortical TRPV5, calbindin-D_28k and NCX1 expressions increased 1.6, 1.5 and 1.2 fold, respectively.

Conclusions

Chronic high-salt diet decreased claudin-2 protein and increased renal TRPV5, calbindin-D_28k, and NCX1. Salt loading is known to reduce the proximal tubular reabsorption of both Na⁺ and Ca²⁺. The reduction in claudin-2 protein expression may be partly responsible for the reduced Ca²⁺ reabsorption in this segment. The concerted upregulation of more distal Ca²⁺-transporting molecules may be a physiological response to curtail the loss of Ca²⁺, although the magnitude of compensation does not seem adequate to bring the urinary Ca²⁺ excretion down to that of the normal-diet group.

Calcium signaling in renal tubular cells

The kidney handles calcium by filtration and reabsorption. About 60% of the plasma calcium is filterable, and 99% is reabsorbed in the tubule. In the proximal tubule, the reabsorption is passive and paracellular, but in the distal tubule is active and transcellular. Thus, renal tubular cells are exposed to very high concentrations of calcium in both, the extracellular and the intracellular compartments. Extracellular calcium signaling is transmitted by the calcium sensing receptor, located both in the luminal and basolateral sides of tubular cells. This receptor is able to control levels of extracellular calcium and acts in consequence to maintain calcium homeostasis. Furthermore, renal tubular cells possess several calcium channels that regulate some of the cell functions. Among those, voltage gated calcium channels, transient receptor potential channels and N-methyl-D-aspartate receptor channels have been reported to control several functions. Those functions include survival, apoptosis, differentiation, epithelial-mesenchymal transition, and active vitamin D and renin synthesis.

Developmental expression of calcium transport proteins in extraembryonic membranes of oviparous and viviparous Zootoca vivipara (Lacertilia, Lacertidae)

The eggshell of oviparous lizards is a significant source of calcium for embryos, whereas the eggshell of viviparous lizards, when present, contains little calcium. In view of the potential cost to embryonic nutrition occasioned by the loss of eggshell calcium, the large number of independent origins of viviparity among lizards is surprising. Concomitant evolution of viviparity and calcium placentotrophy would ameliorate the loss of eggshell calcium, but a mechanism linking these events has yet to be discovered. Zootoca vivipara, a lizard with geographic variation in its mode of parity, is an excellent model for studying mechanisms of calcium transport to oviparous and viviparous embryos because each is highly dependent on calcium secreted by the uterus (eggshell or placenta) and ontogenetic patterns of embryonic calcium mobilization are similar. We compared developmental expression of the calcium transport protein calbindin-D28K in yolk splanchnopleure and chorioallantoic membranes of oviparous and viviparous embryos to test the hypothesis that the mechanism of calcium transport does not differ between modes of parity. We found that the ontogenetic pattern of protein expression is similar between reproductive modes and is correlated with calcium uptake from yolk and either eggshell or placenta. Calbindin-D28K is localized in the chorionic epithelium of embryos of both reproductive modes. These findings suggest that the embryonic calcium transport machinery is conserved in the transition between reproductive modes and that an adaptation of oviparous embryos for calcium uptake from eggshells functions similarly to transport calcium directly from uterine secretions.

Alternative perspective on intestinal calcium absorption: proposed complementary actions of Ca(v)1.3 and TRPV6

Transcellular models of dietary Ca(2+) absorption by the intestine assign essential roles to TRPV6 and calbindin-D(9K) . However, studies with gene-knockout mice challenge this view. Something fundamental is missing. The L-type channel Ca(v) 1.3 is located in the apical membrane from the duodenum to the ileum. In perfused rat jejunum in vivo and in Caco-2 cells, Ca(v) 1.3 mediates sodium glucose transporter 1 (SGLT1)-dependent and prolactin-induced active, transcellular Ca(2+) absorption, respectively. TRPV6 is activated by hyperpolarization and is vitamin D dependent; in contrast, Ca(v) 1.3 is activated by depolarization and is independent of calbindin-D(9K) and vitamin D. This review considers evidence supporting the idea that Ca(v) 1.3 and TRPV6 have complementary roles in the regulation of intestinal Ca(2+) absorption as depolarization and repolarization of the apical membrane occur during and between digestive periods, respectively, and as chyme moves from one intestinal segment to another and food transit times increase. Reassessment of current arguments for paracellular flow reveals that key phenomena have alternative explanations within the integrated Ca(v) 1.3/TRPV6 view of transcellular Ca(2+) absorption.

Characteristic clinical and biochemical profile of recurrent calcium-oxalate nephrolithiasis in patients with metabolic syndrome

BACKGROUND

Metabolic syndrome is a risk factor for nephrolithiasis. This study was performed to evaluate the clinical and biochemical profile of calcium-oxalate nephrolithiasis in stone formers with metabolic syndrome.

METHODS

A total of 526 recurrent stone formers, 184 of them with metabolic syndrome, and 214 controls were examined on a free diet and after a sodium-restricted diet (sodium intake < 100 mmol/24 h).

RESULTS

On free diet, stone formers with metabolic syndrome showed higher sodium excretion [mean (95% confidence interval), 196 (176-218) vs 160 (150-168) mmol/24 h; P < 0.01] and lower citrate excretion [2.23 (1.99-2.58) vs 2.84 (2.51-3.17) mmol/24 h; P < 0.01] compared to controls, whereas stone formers without metabolic syndrome showed higher calcium and oxalate excretion [5.43 (5.01-5.82) vs 3.58 (2.84-4.19) and 0.34 (0.32-0.36) vs 0.26 (0.20-0.31)m mmol/24 h for calcium and oxalate, respectively; P < 0.01] and lower citrate excretion [2.18 (1.98-2.38) vs 2.84 (2.51-3.17) mmol/24 h; P < 0.01] compared to controls. The ion activity product of urinary calcium-oxalate salts was similar between stone formers with and without metabolic syndrome [1.41 (1.31-1.59) vs 1.40 (1.35-1.45); P > 0.05]. After the test diet, this index was lower in diet-compliant stone formers with metabolic syndrome compared to diet-compliant stone formers without metabolic syndrome [1.15 (1.10-1.21) vs 1.39 (1.31-1.45); P < 0.01].

CONCLUSIONS

The biochemical profiles and responses to the sodium-restricted diet were significantly different between stone formers with metabolic syndrome and those without. Dietary habits play a central role in the pathogenesis of nephrolithiasis in stone formers with metabolic syndrome.

Impaired renal calcium absorption in mice lacking calcium channel beta 3 subunits

Transgenic mice lacking calcium channel beta3 subunits (CaVbeta3) were used to determine the involvement of a multimeric calcium channel in mediating stimulated renal calcium absorption. We measured the ability of calcium channel beta3 subunit-null (CaVbeta3-/-) and wild-type (CaVbeta3+/+) mice to increase renal calcium absorption in response to the calcium-sparing diuretic chlorothiazide (CTZ). Control rates of fractional sodium excretion were comparable in CaVbeta3-/- and CaVbeta3+/+ mice and CTZ increased sodium excretion similarly in both groups. CTZ enhanced calcium absorption only in wild-type CaVbeta3+/+ mice. This effect was specific for diuretics acting on distal tubules because both CaVbeta3-/- and CaVbeta3+/+ mice responded comparably to furosemide. The absence of beta3 subunits resulted in compensatory increases of TrpV5 calcium channels, the plasma membrane Ca-ATPase, NCX1 Na/Ca exchanger protein, and calbindin-D9k but not calbindin-D28k. We conclude that TrpV5 mediates basal renal calcium absorption and that a multimeric calcium channel that includes CaVbeta3 mediates stimulated calcium transport.

K+-dependent Na+/Ca2+ exchanger 3 is involved in renal active calcium transport and is differentially expressed in the mouse kidney

Previously, we reported that renal active calcium-transporting genes are highly expressed in female mice and suggested that renal calcium-processing genes play a critical role in normal calcium reabsorption in females (Lee GS, Lee KY, Choi KC, Ryu YH, Paik SG, Oh GT, Jeung EB. J Bone Miner Res 22: 1968-1978, 2007). In the current study, we evaluated the differential expression of renal calcium-processing genes in male and female mice. Using microarray analysis, we identified K(+)-dependent Na(+)/Ca(2+) exchanger 3 (NCKX3) as a gene that was differentially expressed in the kidneys of female and male mice. The expression levels of renal NCKX3 mRNA and protein were higher in female than in male mice, whereas there was no difference between the genders in the levels of NCKX3 expression in the brain. Renal NCKX3 localized to the basolateral layer of distal convoluted tubules, indicating that this protein participates in renal calcium reabsorption. To identify putative regulators in the gender-specific expression of NCKX3, several hormones were injected into mature female and male mice. Although any hormones did not alter NCKX3 expression, adrenal gland-secreted hormones aldosterone and hydrocortisone did downregulate renal NCKX3 mRNA expression in female mice, but they did not change its protein levels. Taken together, the results in this study suggest that a high level of renal NCKX3 expression maintain in distal convoluted tubules may play a role in active calcium transport in the kidneys of female mice.

Parathyroid hormone activates TRPV5 via PKA-dependent phosphorylation

Low extracellular calcium (Ca(2+)) promotes release of parathyroid hormone (PTH), which acts on multiple organs to maintain overall Ca(2+) balance. In the distal part of the nephron, PTH stimulates active Ca(2+) reabsorption via the adenylyl cyclase-cAMP-protein kinase A (PKA) pathway, but the molecular target of this pathway is unknown. The transient receptor potential vanilloid 5 (TRPV5) channel constitutes the luminal gate for Ca(2+) entry in the distal convoluted tubule and has several putative PKA phosphorylation sites. Here, we investigated the effect of PTH-induced cAMP signaling on TRPV5 activity. Using fluorescence resonance energy transfer, we studied cAMP and Ca(2+) dynamics during PTH stimulation of HEK293 cells that coexpressed the PTH receptor and TRPV5. PTH increased cAMP levels, followed by a rise in TRPV5-mediated Ca(2+) influx. PTH (1 to 31) and forskolin, which activate the cAMP pathway, mimicked the stimulation of TRPV5 activity. Remarkably, TRPV5 activation was limited to conditions of strong intracellular Ca(2+) buffering. Cell surface biotinylation studies demonstrated that forskolin did not affect TRPV5 expression on the cell surface, suggesting that it alters the single-channel activity of a fixed number of TRPV5 channels. Application of the PKA catalytic subunit, which phosphorylated TRPV5, directly increased TRPV5 channel open probability. Alanine substitution of threonine-709 abolished both in vitro phosphorylation and PTH-mediated stimulation of TRPV5. In summary, PTH activates the cAMP-PKA signaling cascade, which rapidly phosphorylates threonine-709 of TRPV5, increasing the channel's open probability and promoting Ca(2+) reabsorption in the distal nephron.

Endocrine regulation of calcium transport in epithelia

1. Calcium (re)absorption occurs in epithelia, including the intestine, kidney, mammary glands, placenta and gills (in the case of fish). 2. Calcium is transported across epithelia by two transport mechanisms, paracellular and transcellular, and the movement is regulated by a complex array of transport processes that are mediated by hormonal, developmental and physiological factors involving the gastrointestinal tract, bone, kidney and the parathyroids. 3. Clear understanding of the calcium transport pathways and their endocrine regulation is critical for minimizing various metabolic and health disorders at different physiological stages. Here, we first briefly review the calcium transport mechanisms before discussing in detail the endocrine factors that regulate calcium transport in the epithelia.

Molecular characterization of an epithelial Ca2+ channel-like gene from crayfish Procambarus clarkii

This study describes the cloning, sequencing and functional characterization of an epithelial Ca(2+) channel (ECaC)-like gene isolated from antennal gland (kidney) of the freshwater crayfish Procambarus clarkii. The full-length cDNA consisted of 2687 bp with an open reading frame of 2169 bp encoding a protein of 722 amino acids with a predicted molecular mass of 81.7 kDa. Crayfish ECaC had 76-78% identity at the mRNA level (80-82% amino acid identity) with published fish sequences and 56-62% identity at the mRNA level (52-60% amino acid identity) with mammalian ECaCs. Secondary structure of the crayfish ECaC closely resembled that of cloned ECaCs. Postmolt ECaC expression was exclusively restricted to epithelia associated with Ca(2+) influx and was virtually undetectable in non-epithelial tissues (eggs, muscle). Compared with expression levels in hepatopancreas, expression in gill was 10-fold greater and expression was highest in antennal gland (15-fold greater than in hepatopancreas). Compared with baseline expression levels in intermolt stage, expression of ECaC in antennal gland increased 7.4- and 23.8-fold, respectively, in pre- and postmolt stages of the molting cycle. This increase was localized primarily in the labyrinth and nephridial canal, regions of the antennal gland associated with renal Ca(2+) reabsorption. The ECaC in crayfish appears to be expressed in epithelia associated with unidirectional Ca(2+) influx and relative expression is correlated with rate of Ca(2+) influx.

Distribution of vitamin D3 receptor in the epididymal region of roosters (Gallus domesticus) is cell and segment specific

Vitamin D3 is a steroid hormone well known by its role in maintaining calcium homeostasis, however this hormone may also participate in other biological functions, including control of reproductive processes. The vitamin D3 action is mediated by the vitamin D3 receptor (VDR). VDR is widely distributed in the rodent reproductive tract, however the occurrence of VDR and the role of the vitamin D3 in the avian reproductive tract remain unknown. The aim of the present study was to investigate the expression and cellular distribution of VDR in the epididymal region of roosters. VDR expression was investigated by Western blotting analysis and the tissue distribution of the receptor was determined by immunohistochemistry. The Western blotting assay revealed a major VDR protein band of 61kDa in the epididymal region of rooster. Nuclear VDR expression was found in all segments of the epididymal region, namely rete testis, efferent ductules, connecting ducts and epididymal ducts. Nonciliated cells of the distal efferent ductules showed the highest levels of VDR expression, followed by the proximal efferent ductules and rete testis. The connecting and epididymal ducts showed less intense VDR immunostaining. The differential VDR expression in the epididymal region segments reveals that several extratesticular ducts may be target for vitamin D3 action and suggests that vitamin D3 may have a regional-specific function, such as calcium transport, that is modulated through VDR activity.

Duodenal calcium absorption in dexamethasone-treated mice: functional and molecular aspects

Reduced intestinal calcium absorption may be part of the pathogenesis of glucocorticoid-induced osteoporosis. 1,25(OH)2D3 is the major regulator of the expression of the active duodenal calcium absorption genes: TRPV6 (influx), calbindin-D9K (intracellular transfer) and PMCA1b (extrusion). We investigated the influence of dexamethasone (5 days: 2 mg/kg bw) on calcium absorption in vivo and on the expression of intestinal and renal calcium transporters in calcium-deprived mice. Total and free 1,25(OH)2D3-concentrations were halved, in line with decreased 25(OH)D3-1-alpha-hydroxylase and increased 24-hydroxylase expression. Nevertheless, no difference in duodenal or renal calcium transporter expression pattern could be detected between vehicle and dexamethasone-treated mice. Accordingly, dexamethasone did not affect in vivo calcium absorption. By contrast, increased calcemia and collagen C-terminal telopeptide levels reflected increased bone resorption. Decreased osteocalcin levels suggested impaired bone formation. Hence, short-term glucocorticoid excess in young animals affected bone metabolism without detectable changes in intestinal or renal calcium handling.

Critical Role of the Epithelial Ca2+Channel TRPV5 in Active Ca2+Reabsorption as Revealed by TRPV5/Calbindin-D28KKnockout Mice

The epithelial Ca(2+) channel TRPV5 facilitates apical Ca(2+) entry during active Ca(2+) reabsorption in the distal convoluted tubule. In this process, cytosolic Ca(2+) remains at low nontoxic concentrations because the Ca(2+) influx is buffered rapidly by calbindin-D(28K). Subsequently, Ca(2+) that is bound to calbindin-D(28K) is shuttled toward the basolateral Ca(2+) extrusion systems. For addressing the in vivo role of TRPV5 and calbindin-D(28K) in the maintenance of the Ca(2+) balance, single- and double-knockout mice of TRPV5 and calbindin-D(28K) (TRPV5(-/-), calbindin-D(28K)(-/-), and TRPV5(-/-)/calbindin-D(28K)(-/-)) were characterized. These mice strains were fed two Ca(2+) diets (0.02 and 2% wt/wt) to investigate the influence of dietary Ca(2+) content on the Ca(2+) balance. Urine analysis indicated that TRPV5(-/-)/calbindin-D(28K)(-/-) mice exhibit on both diets hypercalciuria compared with wild-type mice. Ca(2+) excretion in TRPV5(-/-)/calbindin-D(28K)(-/-) mice was not significantly different from TRPV5(-/-) mice, whereas calbindin-D(28K)(-/-) mice did not show hypercalciuria. The similarity between TRPV5(-/-)/calbindin-D(28K)(-/-) and TRPV5(-/-) mice was supported further by an equivalent increase in renal calbindin-D(9K) expression and in intestinal Ca(2+) hyperabsorption as a result of upregulation of calbindin-D(9K) and TRPV6 expression in the duodenum. Elevated serum parathyroid hormone and 1,25-dihydroxyvitamin D(3) levels accompanied the enhanced expression of the Ca(2+) transporters. Intestinal Ca(2+) absorption and expression of calbindin-D(9K) and TRPV6, as well as serum parameters of the calbindin-D(28K)(-/-) mice, did not differ from those of wild-type mice. These results underline the gatekeeper function of TRPV5 being the rate-limiting step in active Ca(2+) reabsorption, unlike calbindin-D(28K), which possibly is compensated by calbindin-D(9K).

Recent advances in physiological calcium homeostasis

A constant extracellular Ca2+ concentration is required for numerous physiological functions at tissue and cellular levels. This suggests that minor changes in Ca2+ will be corrected by appropriate homeostatic systems. The system regulating Ca2+ homeostasis involves several organs and hormones. The former are mainly the kidneys, skeleton, intestine and the parathyroid glands. The latter comprise, amongst others, the parathyroid hormone, vitamin D and calcitonin. Progress has recently been made in the identification and characterisation of Ca2+ transport proteins CaT1 and ECaC and this has provided new insights into the molecular mechanisms of Ca2+ transport in cells. The G-protein coupled calcium-sensing receptor, responsible for the exquisite ability of the parathyroid gland to respond to small changes in serum Ca2+ concentration was discovered about a decade ago. Research has focussed on the molecular mechanisms determining the serum levels of 1,25(OH)2D3, and on the transcriptional activity of the vitamin D receptor. The aim of recent work has been to elucidate the mechanisms and the intracellular signalling pathways by which parathyroid hormone, vitamin D and calcitonin affect Ca2+ homeostasis. This article summarises recent advances in the understanding and the molecular basis of physiological Ca2+ homeostasis.

Renal Ca2+ handling in sgk1 knockout mice

Coexpression studies in Xenopus oocytes revealed the ability of the serum- and glucocorticoid-inducible kinase 1 (SGK1) to stimulate the renal epithelial Ca(2+) channel TRPV5. SGK1 increases the abundance of the channel protein in the plasma membrane, an effect requiring the participation of the Na(+)/H(+) exchanger regulating factor 2 (NHERF2). The present study was performed to explore the role of SGK1 in the regulation of renal Ca(2+) handling in vivo. To this end, TRPV5, calbindin D-28K abundance, and renal Ca(2+) excretion were analyzed in gene-targeted mice lacking functional SGK1 (sgk1( -/- )) and their age- and sex-matched littermates (sgk1( +/+ )). Immunohistochemistry revealed lower abundance of TRPV5 and calbindin D-28K protein in sgk1( -/- ) mice than in sgk1( +/+ ) mice, both fed with control diet. Feeding the mice a Ca(2+)-deficient diet marked ly increased TRPV5 protein abundance in both genotypes. Renal Ca(2+) excretion under control diet was significantly lower in sgk1 ( -/- ) than in sgk1( +/+ ) mice. The Ca(2+)-deficient diet decreased renal excretion of Ca(2+) to the same levels in both phenotypes. Furosemide increased fractional Ca(2+) excretion and dissipated the difference between phenotypes. We conclude that lack of SGK1 may lead to decrease in TRPV5 abundance in connecting tubules but does not abrogate TRPV5 regulation. The decrease in abundance of TRPV5 in connecting tubules of sgk1( -/- ) mice is presumably compensated for by enhanced Ca(2+) reabsorption in upstream nephron segments such as the loop of Henle, which may indirectly result from impaired SGK1-dependent Na(+) reabsorption in the aldosterone-sensitive distal part of the nephron, salt loss, and enhanced Na(+) (and Ca(2+)) reabsorption in those upstream nephron segments.

Acid-base status determines the renal expression of Ca2+ and Mg2+ transport proteins

Chronic metabolic acidosis results in renal Ca2+ and Mg2+ wasting, whereas chronic metabolic alkalosis is known to exert the reverse effects. It was hypothesized that these adaptations are mediated at least in part by the renal Ca2+ and Mg2+ transport proteins. The aim of this study, therefore, was to determine the effect of systemic acid-base status on renal expression of the epithelial Ca2+ channel TRPV5, the Ca2+-binding protein calbindin-D28K, and the epithelial Mg2+ channel TRPM6 in relation to Ca2+ and Mg2+ excretion. Chronic metabolic acidosis that was induced by NH4Cl loading or administration of the carbonic anhydrase inhibitor acetazolamide for 6 d enhanced calciuresis accompanied by decreased renal TRPV5 and calbindin-D28K mRNA and protein abundance in wild-type mice. In contrast, metabolic acidosis did not affect Ca2+ excretion in TRPV5 knockout (TRPV5-/-) mice, in which active Ca2+ reabsorption is effectively abolished. This demonstrates that downregulation of renal Ca2+ transport proteins is responsible for the hypercalciuria. Conversely, chronic metabolic alkalosis that was induced by NaHCO3 administration for 6 d increased the expression of Ca2+ transport proteins accompanied by diminished urine Ca2+ excretion in wild-type mice. However, this Ca2+-sparing action persisted in TRPV5-/- mice, suggesting that additional mechanisms apart from upregulation of active Ca2+ transport contribute to the hypocalciuria. Furthermore, chronic metabolic acidosis decreased renal TRPM6 expression, increased Mg2+ excretion, and decreased serum Mg2+ concentration, whereas chronic metabolic alkalosis resulted in the exact opposite effects. In conclusion, these data suggest that regulation of Ca2+ and Mg2+ transport proteins contributes importantly to the effects of acid-base status on renal divalent handling.

Vitamin D

The vitamin D endocrine system plays an essential role in calcium homeostasis and bone metabolism, but research during the past two decades has revealed a diverse range of biological actions that include induction of cell differentiation, inhibition of cell growth, immunomodulation, and control of other hormonal systems. Vitamin D itself is a prohormone that is metabolically converted to the active metabolite, 1,25-dihydroxyvitamin D [1,25(OH)(2)D]. This vitamin D hormone activates its cellular receptor (vitamin D receptor or VDR), which alters the transcription rates of target genes responsible for the biological responses. This review focuses on several recent developments that extend our understanding of the complexities of vitamin D metabolism and actions: the final step in the activation of vitamin D, conversion of 25-hydroxyvitamin D to 1,25(OH)(2)D in renal proximal tubules, is now known to involve facilitated uptake and intracellular delivery of the precursor to 1alpha-hydroxylase. Emerging evidence using mice lacking the VDR and/or 1alpha-hydroxylase indicates both 1,25(OH)(2)D(3)-dependent and -independent actions of the VDR as well as VDR-dependent and -independent actions of 1,25(OH)(2)D(3). Thus the vitamin D system may involve more than a single receptor and ligand. The presence of 1alpha-hydroxylase in many target cells indicates autocrine/paracrine functions for 1,25(OH)(2)D(3) in the control of cell proliferation and differentiation. This local production of 1,25(OH)(2)D(3) is dependent on circulating precursor levels, providing a potential explanation for the association of vitamin D deficiency with various cancers and autoimmune diseases.

Vitamin D receptor is required for dietary calcium-induced repression of calbindin-D9k expression in mice

Calbindin (CaBP), the vitamin D-dependent calcium-binding protein, is believed to play an important role in intracellular calcium transport. The aim of this study was to investigate the effect of high dietary calcium on the expression of CaBP-D9k and CaBP-D28k in the presence and absence of a functional vitamin D receptor (VDR). Treatment with the HCa-Lac diet containing 2% calcium, 1.5% phosphorus and 20% lactose reversed the hypocalcemia seen in adult VDR-null mice in 3 weeks but did not significantly change the blood ionized calcium in wild-type mice. This dietary treatment dramatically suppressed both the duodenal and the renal CaBP-D9k expression in wild-type mice at both mRNA and protein levels but had little effect on the expression of the same gene in VDR-null mice. Removal of this diet gradually restored the expression of CaBP-D9k to the untreated level in wild-type mice. Only moderate or little change in CaBP-D28k expression was seen in wild-type and VDR-null mice fed with the HCa-Lac diet. The VDR content in the duodenum or kidney of wild-type mice was not altered by the dietary treatment. These results suggest that calcium regulates CaBP-D9k expression by modulating the circulating 1,25-dihydrxyvitamin D(3) level and that VDR is thus required for the dietary calcium-induced suppression of CaBP-D9k expression. Calcium regulation of the CaBP-D9k level may represent an important mechanism by which animals maintain their calcium balance.

Microscopical and functional aspects of calcium-transport and deposition in terrestrial isopods

Terrestrial isopods (Crustacea) are excellent model organisms to study epithelial calcium-transport and the regulation of biomineralization processes. They molt frequently and resorb cuticular CaCO(3) before the molt to prevent excessive loss of Ca(2+) ions when the old cuticle is shed. The resorbed mineral is stored in CaCO(3) deposits within the ecdysial gap of the first four anterior sternites. After the molt, the deposits are quickly resorbed to mineralise the posterior part of the new cuticle. The deposits contain numerous small spherules composed of an organic matrix and amorphous CaCO(3), which has a high solubility and, therefore, facilitates quick mobilization of Ca(2+) and HCO(3)(-) ions. During the formation and resorption of the deposits large amounts of Ca(2+), HCO(3)(-) and H(+) are transported across the anterior sternal epithelial cells. Within the last years, various light and electron microscopical techniques have been used to characterize the CaCO(3) deposits and the cellular mechanisms involved in biomineralization. The work on the CaCO(3) deposits includes studies on the ultrastructure of the deposits, the sequence of events during deposit formation and dissolution, and the mineral composition of the sternal deposits. The differentiation of the anterior sternal epithelial cells and the mechanisms of epithelial ion transport required for the mineralization and demineralisation of the deposits was studied using various analytical light and electron microscopical techniques including polarized light microscopy, immunocytochemistry, electron microprobe analysis, electron energy loss spectroscopy and electron spectroscopic imaging. Comparative analysis of deposit morphology and the differentiation of the sternal epithelia provide information on the evolution of CaCO(3) deposit formation in relation to the degree of adaptation to terrestrial environments.

Calcium absorption across epithelia

Ca(2+) is an essential ion in all organisms, where it plays a crucial role in processes ranging from the formation and maintenance of the skeleton to the temporal and spatial regulation of neuronal function. The Ca(2+) balance is maintained by the concerted action of three organ systems, including the gastrointestinal tract, bone, and kidney. An adult ingests on average 1 g Ca(2+) daily from which 0.35 g is absorbed in the small intestine by a mechanism that is controlled primarily by the calciotropic hormones. To maintain the Ca(2+) balance, the kidney must excrete the same amount of Ca(2+) that the small intestine absorbs. This is accomplished by a combination of filtration of Ca(2+) across the glomeruli and subsequent reabsorption of the filtered Ca(2+) along the renal tubules. Bone turnover is a continuous process involving both resorption of existing bone and deposition of new bone. The above-mentioned Ca(2+) fluxes are stimulated by the synergistic actions of active vitamin D (1,25-dihydroxyvitamin D(3)) and parathyroid hormone. Until recently, the mechanism by which Ca(2+) enter the absorptive epithelia was unknown. A major breakthrough in completing the molecular details of these pathways was the identification of the epithelial Ca(2+) channel family consisting of two members: TRPV5 and TRPV6. Functional analysis indicated that these Ca(2+) channels constitute the rate-limiting step in Ca(2+)-transporting epithelia. They form the prime target for hormonal control of the active Ca(2+) flux from the intestinal lumen or urine space to the blood compartment. This review describes the characteristics of epithelial Ca(2+) transport in general and highlights in particular the distinctive features and the physiological relevance of the new epithelial Ca(2+) channels accumulating in a comprehensive model for epithelial Ca(2+) absorption.

Requirement of PDZ Domains for the Stimulation of the Epithelial Ca2+ Channel TRPV5 by the NHE Regulating Factor NHERF2 and the Serum and Glucocorticoid Inducible Kinase SGK1

Renal calcium reabsorption involves the epithelial calcium channel ECaC1 (TRPV5) which is tightly regulated by 1,25(OH)2D3. As shown recently, TRPV5 is activated by the serum and glucocorticoid inducible kinase SGK1, a kinase transcriptionally upregulated by 1,25(OH)2D3. This stimulatory effect is due to enhanced TRPV5 abundance in the plasma membrane and requires the presence of the scaffold protein NHERF2 (sodium hydrogen exchanger regulating factor 2). The present study aims to define the molecular requirements for the interaction of TRPV5 with SGK1 and NHERF2. Pull-down experiments and overlay assays revealed that the TRPV5 C-tail interacts in a Ca2+-independent manner with NHERF2. Deletion of the second but not of the first PDZ domain in NHERF2 abrogates the stimulating effect of SGK1/NHERF2 on TRPV5 protein abundance in the plasma membrane as quantified by chemiluminescence and electrophysiology. Thus, the second PDZ domain in NHERF2 is required for stabilization at or TRPV5 targeting to the plasma membrane. The experiments demonstrate the significance of SGK1 and NHERF2 as TRPV5 modulators which are likely to participate in the regulation of calcium homeostasis by 1,25(OH)2D3.

Expression of calbindin-D28K by yolk sac and chorioallantoic membranes of the corn snake,Elaphe guttata

The yolk splanchnopleure and chorioallantoic membrane of oviparous reptiles transport calcium from the yolk and eggshell to the developing embryo. Among oviparous amniotes, the mechanism of calcium mobilization to embryos has been studied only in domestic fowl, in which the mechanism of calcium transport of the yolk splanchnopleure differs from the chorioallantoic membrane. Transport of calcium is facilitated by calbindin-D(28K) in endodermal cells of the yolk splanchnopleure of chickens but the chorioallantoic membrane does not express calbindin-D(28K). We used immunoblotting to assay for calbindin-D(28K) expression in yolk splanchnopleure and chorioallantoic membrane of the corn snake, Elaphe guttata, to test the hypothesis that the mechanism of calcium transport by extraembryonic membranes of snakes is similar to birds. High calbindin-D(28K) expression was detected in samples of yolk splanchnopleure and chorioallantoic membrane during late embryonic stages. We conclude that calbindin-D(28K) is expressed in these extraembryonic membranes to facilitate transport of calcium and that the mechanism of calcium transport of the chorioallantoic membrane of the corn snake differs from that of the chicken. Further, we conclude that calbindin-D(28K) expression is developmentally regulated and increases during later embryonic stages in the corn snake.

A role for Ca(v)1.3 in rat intestinal calcium absorption

Active Ca(2+) absorption through epithelial Ca(2+) channels TRPV5/6 in duodenum is activated by hyperpolarisation. However, when diet and Ca(2+) are plentiful, digestion products cause depolarisation. We therefore used homology-based PCR from a rat jejunal mucosal cDNA preparation to reveal the presence of the neuroendocrine L-type isoform Ca(v)1.3alpha(1). Immunocytochemical labelling and immunoblotting localised Ca(v)1.3 alpha(1) protein in apical membrane from proximal jejunum to mid ileum. Perfusion studies in vivo with 1.25 mM luminal Ca(2+) revealed L-type channel activity. Inhibition of glucose absorption with phloridzin strongly inhibited 45Ca(2+) absorption; absorption was inhibited by nifedipine and Mg(2+) and activated by Bay K 8644, none of which affect TRPV5/6. At 10mM Ca(2+), nifedipine inhibited 45Ca(2+) absorption with a time course similar to that at 1.25 mM Ca(2+): absorption was therefore channel-mediated rather than paracellular. We suggest that in times of dietary sufficiency, Ca(v)1.3 may mediate a significant route of Ca(2+) absorption into the body.

Vitamin D3upregulates plasma membrane Ca2+-ATPase expression and potentiates apico-basal Ca2+flux in MDCK cells

Plasma membrane Ca2+-ATPases (PMCAs) are a ubiquitous system for the expulsion of Ca2+from eukaryotic cells. In tight monolayers of polarized Madin-Darby canine kidney (MDCK) cells representing a distal kidney tubule model, PMCAs are responsible for about one-third of the vectorial Ca2+transport under resting conditions, with the remainder being provided by the Na+/Ca2+exchanger. Vitamin D3(VitD) is known to increase PMCA expression and activity in Ca2+-transporting tissues such as the intestine, as well as in osteoblasts and Madin-Darby bovine kidney epithelial cells. We found that VitD upregulated the expression of the PMCAs (mainly PMCA4b) in MDCK cell lysates at the RNA and protein level in a time- and dose-dependent manner. Interestingly, VitD caused a decrease of the PMCAs in the apical plasma membrane fraction and a concomitant increase of the pumps in the basolateral membrane. Functional studies demonstrated that transcellular45Ca2+flux from the apical-to-basolateral compartment was significantly enhanced by VitD. These findings demonstrate that VitD is a positive regulator of the PMCAs in MDCK epithelial cells. The correlation of decreased apical/increased basolateral expression of the PMCAs with an increase in transcellular Ca2+flux from the apical (urine) toward the basolateral (blood) compartment indicates the physiological relevance of VitD function in kidney tubular Ca2+reabsorption.

Characterization of a murine renal distal convoluted tubule cell line for the study of transcellular calcium transport

To unravel the molecular regulation of renal transcellular Ca(2+) transport, a murine distal convoluted tubule (mpkDCT) cell line derived from distal convoluted tubules (DCT) microdissected from a SV-PK/Tag transgenic mouse was characterized. This cell line originated from DCT only, as mRNA encoding for the DCT marker thiazide-sensitive Na(+)/Cl(-) cotransporter was expressed, whereas mRNA encoding for the connecting tubule and collecting duct marker aquaporin-2 was not detected, as determined by reverse-transcriptase PCR. mpkDCT cells expressed mRNA encoding the Ca(2+) channels TRPV5 and TRPV6 and other key players necessary for transcellular Ca(2+) transport, i.e., calbindin-D(9k), calbindin-D(28k), plasma membrane Ca(2+)-ATPase isoform 1b, and Na(+)/Ca(2+) exchanger 1. Primary cultures of DCT cells exhibited net transcellular Ca(2+) transport of 0.4 +/- 0.1 nmol.h(-1).cm(-2), whereas net transcellular Ca(2+) transport across mpkDCT cells was significantly higher at 2.4 +/- 0.4 nmol.h(-1).cm(-2). Transcellular Ca(2+) transport across mpkDCT cells was completely inhibited by ruthenium red, an inhibitor of TRPV5 and TRPV6, but not by the voltage-operated Ca(2+) channel inhibitors felodipine and verapamil. With the use of patch-clamp analysis, the IC(50) of ruthenium red on Na(+) currents was between the values measured for TRPV5- and TRPV6-expressing HEK 293 cells, suggesting that TRPV5 and/or TRPV6 is possibly active in mpkDCT cells. Forskolin in combination with IBMX, 1,25-dihydroxyvitamin D(3), and 1-deamino-8-d-arginine vasopressin increased transcellular Ca(2+) transport, whereas PMA and parathyroid hormone had no significant effect. In conclusion, the murine mpkDCT cell line provides a unique cell model in which to study the molecular regulation of transcellular Ca(2+) transport in the kidney in vitro.

Intestinal calcium transporter genes are upregulated by estrogens and the reproductive cycle through vitamin D receptor-independent mechanisms

1alpha,25(OH)2-vitamin D strongly regulates the expression of the epithelial calcium channel CaT1. CaT1 expression is reduced in ERKOalpha mice and induced by estrogen treatment, pregnancy, or lactation in VDR WT and KO mice. Estrogens and vitamin D are thus independent potent regulators of the expression of this calcium influx mechanism, which is involved in active intestinal calcium absorption.

INTRODUCTION

Active duodenal calcium absorption consists of three major steps: calcium influx into, transfer through, and extrusion out of the enterocyte. These steps are carried out by the calcium transport protein 1 (CaT1), calbindin-D9K, and the plasma membrane calcium ATPase (PMCA1b), respectively. We investigated whether estrogens or hormonal changes during the female reproductive cycle influence the expression of these genes, and if so, whether these effects are vitamin D-vitamin D receptor (VDR) dependent.

MATERIALS AND METHODS

We evaluated duodenal expression patterns in estrogen receptor (ER)alpha and -beta knockout (KO) mice, as well as in ovariectomized, estrogen-treated, pregnant, and lactating VDR wild-type (WT) and VDR KO mice.

RESULTS

Expression of calcium transporter genes was not altered in ERKObeta mice. CaT1 mRNA expression was reduced by 55% in ERKOalpha mice, while the two other calcium transporter genes were not affected. Ovariectomy caused no change in duodenal expression pattern of VDR WT and KO mice, whereas treatment with a pharmacologic dose of estrogens induced CaT1 mRNA expression in VDR WT (4-fold) and KO (8-fold) mice. Pregnancy enhanced CaTI expression equally in VDR WT and KO mice (12-fold). Calbindin-D9K and PMCA1b expression increased to a lesser extent and solely in pregnant VDR WT animals. In lactating VDR WT and KO mice, CaT1 mRNA expression increased 13 times, which was associated with a smaller increase in calbindin-D9K protein content and PMCA1b mRNA expression.

CONCLUSIONS

Estrogens or hormonal changes during pregnancy or lactation have distinct, vitamin D-independent effects at the genomic level on active duodenal calcium absorption mechanisms, mainly through a major upregulation of the calcium influx channel CaT1. The estrogen effects seem to be mediated solely by ERalpha.

(Patho)physiological implications of the novel epithelial Ca2+ channels TRPV5 and TRPV6

The epithelial Ca(2+) channels TRPV5 and TRPV6 constitute the apical Ca(2+) entry mechanism in active Ca(2+) (re)absorption. These two members of the superfamily of transient receptor potential (TRP) channels were cloned from the vitamin-D-responsive epithelia of kidney and small intestine and subsequently identified in other tissues such as bone, pancreas and prostate. These channels are regulated by vitamin D as exemplified in animal models of vitamin-D-deficiency rickets. In addition, the epithelial Ca(2+) channels might be involved in the multifactorial pathogenesis of disorders ranging from idiopathic hypercalciuria, stone disease and postmenopausal osteoporosis. This review highlights the emerging (patho)physiological implications of these epithelial Ca(2+) channels.

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.

Disruption of the caveolin-1 gene impairs renal calcium reabsorption and leads to hypercalciuria and urolithiasis

Using LoxP/Cre technology, we generated a knockout mouse homozygous for a null mutation in exon 2 of Cav1. In male Cav1-/- animals, we observed a dramatic increase in the incidence of urinary calcium stone formation. In 5-month-old male mice, the incidence of early urinary calculi was 67% in Cav1-/- mice compared to 19% in Cav1+/+ animals. Frank stone formation was observed in 13% of Cav1-/- males but was not seen in Cav1+/+ mice. Urine calcium concentration was significantly higher in Cav1-/- male mice compared to Cav1+/+ mice. In Cav1-/- mice, distal convoluted tubule cells were completely devoid of Cav1 and the localization of plasma membrane calcium ATPase was disrupted. Functional studies confirmed that active calcium absorption was significantly reduced in Cav1-/- compared to Cav1+/+ male mice. These results demonstrate that disruption of the Cav1 gene promotes the progressive steps required for urinary calcium stone formation and establish a new mouse model for urinary stone disease.

Chloride channels in the kidney: lessons learned from knockout animals

Cl- channels are involved in a range of functions, including regulation of cell volume and/or intracellular pH, acidification of intracellular vesicles, and vectorial transport of NaCl across many epithelia. Numerous Cl- channels have been identified in the kidney, based on single-channel properties such as conductance, anion selectivity, gating, and response to inhibitors. The molecular counterpart of many of these Cl- channels is still not known. This review will focus on gene-targeted mouse models disrupting two structural classes of Cl- channels that are relevant for the kidney: the CLC family of voltage-gated Cl- channels and the CFTR. Disruption of several members of the CLC family in the mouse provided useful models for various inherited diseases of the kidney, including Dent's disease and diabetes insipidus. Mice with disrupted CFTR are valuable models for cystic fibrosis (CF), the most common autosomal recessive, lethal disease in Caucasians. Although CFTR is expressed in various nephron segments, there is no overt renal phenotype in CF. Analysis of CF mice has been useful to identify the role and potential interactions of CFTR in the kidney. Furthermore, observations made in CF mice are potentially relevant to all other models of Cl- channel knockouts because they emphasize the importance of alternative Cl- pathways in such models.

Effects of extracellular purines on ion transport across the integument of Hirudo medicinalis

Little is known about the long-term regulation of epithelial ion transport in invertebrates and the specific mediators involved. For some years, we have been investigating the short-term regulation of transepithelial ion transport across the dorsal integument of the leech Hirudo medicinalis, and we have established a model of Na+ uptake. In the present study, we investigated the effect of long-term acclimation on transintegumental ion transport by adapting leeches to high-salinity conditions. We dissected segments of dorsal integument and measured ion currents in Ussing chamber experiments. Electrophysiological variables, such as transepithelial potential(VT) and short-circuit-current (Isc),were profoundly affected by adaptation to high-salinity conditions. The total transepithelial Na+ current (INa) decreased from 7.66±0.82 to 4.6±0.54 μA cm-2 in preparations adapted to high salinity. The involvement of epithelial Na+channels was determined as current inhibition (Iami) by apical application of amiloride; Na+ channels were equally active in control epithelia and epithelia from leeches adapted to high salinity. Removal of Ca2+ from the apical solutions, which is believed to reduce intracellular Ca2+ concentrations, equalized transepithelial variables between high-salt-adapted integuments and control integuments.

Extracellular purines regulate transepithelial Cl- secretion and Na+ absorption. In a variety of tissues we tested ATP and adenosine for their effects on epithelial transport. Examination of integuments from pondwater- and high-salinity-adapted leeches revealed different sensitivities for these purines. Apical and basolateral application of ATP both stimulated transepithelial Na+ uptake and Iami. Adenosine upregulated non-Na+ currents and acted from the basolateral side only. Apical Ca2+-free conditions attenuated these effects of purines on transepithelial currents. Extracellular UTP had no effect on ion transport.

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.

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.

Human cortical distal nephron: distribution of electrolyte and water transport pathways

The exact distributions of the different salt transport systems along the human cortical distal nephron are unknown. Immunohistochemistry was performed on serial cryostat sections of healthy parts of tumor nephrectomized human kidneys to study the distributions in the distal convolution of the thiazide-sensitive Na-Cl cotransporter (NCC), the beta subunit of the amiloride-sensitive epithelial Na channel (ENaC), the vasopressin-sensitive water channel aquaporin 2 (AQP2), and aquaporin 3 (AQP3), the H(+) ATPase, the Na-Ca exchanger (NCX), plasma membrane calcium-ATPase, and calbindin-D28k (CaBP). The entire human distal convolution and the cortical collecting duct (CCD) display calbindin-D28k, although in variable amounts. Approximately 30% of the distal convolution profiles reveal NCC, characterizing the distal convoluted tubule. NCC overlaps with ENaC in a short portion at the end of the distal convoluted tubule. ENaC is displayed all along the connecting tubule (70% of the distal convolution) and the CCD. The major part of the connecting tubule and the CCD coexpress aquaporin 2 with ENaC. Intercalated cells, undetected in the first 20% of the distal convolution, were interspersed among the segment-specific cells of the remainder of the distal convolution, and of the CCD. The basolateral calcium extruding proteins, Na-Ca exchanger (NCX), and the plasma membrane Ca(2+)-ATPase were found all along the distal convolution, and, in contrast to other species, along the CCD, although in varying amounts. The knowledge regarding the precise distribution patterns of transport proteins in the human distal nephron and the knowledge regarding the differences from that in laboratory animals may be helpful for diagnostic purposes and may also help refine the therapeutic management of electrolyte disorders.

The TRP family of cation channels: probing and advancing the concepts on receptor-activated calcium entry

Stimulation of membrane receptors linked to a phospholipase C and the subsequent production of the second messengers diacylglycerol and inositol-1,4,5-trisphosphate (InsP(3)) is a signaling pathway of fundamental importance in eukaryotic cells. Signaling downstream of these initial steps involves mobilization of Ca(2+) from intracellular stores and Ca(2+) influx through the plasma membrane. For this influx, several contrasting mechanisms may be responsible but particular relevance is attributed to the induction of Ca(2+) influx as consequence of depletion of intracellular calcium stores. This phenomenon (frequently named store-operated calcium entry, SOCE), in turn, may be brought about by various signals, including soluble cytosolic factors, interaction of proteins of the endoplasmic reticulum with ion channels in the plasma membrane, and a secretion-like coupling involving translocation of channels to the plasma membrane. Experimental approaches to analyze these mechanisms have been considerably advanced by the discovery of mammalian homologs of the Drosophila cation channel transient receptor potential (TRP). Some members of the TRP family can be expressed to Ca(2+)-permeable channels that enable SOCE; other members form channels activated independently of stores. TRP proteins may be an essential part of endogenous Ca(2+) entry channels but so far expression of most TRP cDNAs has not resulted in restitution of channels found in any mammalian cells, suggesting the requirement for further unknown subunits. A major exception is CaT1, a TRP channel demonstrated to provide Ca(2+)-selective, store-operated currents identical to those characterized in several cell types. Ongoing and future research on TRP channels will be crucial to understand the molecular basis of receptor-mediated Ca(2+) entry, with respect to the structure of the entry channels as well as to the mechanisms of its activation and regulation.

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.

Distribution of transcellular calcium and sodium transport pathways along mouse distal nephron

The organization of Na(+) and Ca(2+) transport pathways along the mouse distal nephron is incompletely known. We revealed by immunohistochemistry a set of Ca(2+) and Na(+) transport proteins along the mouse distal convolution. The thiazide-sensitive Na(+)-Cl(-) cotransporter (NCC) characterized the distal convoluted tubule (DCT). The amiloride-sensitive epithelial Na(+) channel (ENaC) colocalized with NCC in late DCT (DCT2) and extended to the downstream connecting tubule (CNT) and collecting duct (CD). In early DCT (DCT1), the basolateral Ca(2+)-extruding proteins [Na(+)/Ca(2+) exchanger (NCX), plasma membrane Ca(2+)-ATPase (PCMA)] and the cytoplasmic Ca(2+)-binding protein calbindin D(28K) (CB) were found at very low levels, whereas the cytoplasmic Ca(2+)/Mg(2+)-binding protein parvalbumin was highly abundant. NCX, PMCA, and CB prevailed in DCT2 and CNT, where we located the apical epithelial Ca(2+) channel (ECaC1). Its subcellular localization changed from apical in DCT2 to exclusively cytoplasmic at the end of CNT. NCX and PMCA decreased in parallel with the fading of ECaC1 in the apical membrane. All three of them were undetectable in CD. These findings disclose DCT2 and CNT as major sites for transcellular Ca(2+) transport in the mouse distal nephron. Cellular colocalization of Ca(2+) and Na(+) transport pathways suggests their mutual interactions in transport regulation.