Modulation of renal Ca2+ transport protein genes by dietary Ca2+ and 1,25-dihydroxyvitamin D3 in 25-hydroxyvitamin D3-1alpha-hydroxylase knockout mice

Coordinated regulation of TRPV5-mediated Ca²⁺ transport in primary distal convolution cultures

Fine-tuning of renal calcium ion (Ca(2+)) reabsorption takes place in the distal convoluted and connecting tubules (distal convolution) of the kidney via transcellular Ca(2+) transport, a process controlled by the epithelial Ca(2+) channel Transient Receptor Potential Vanilloid 5 (TRPV5). Studies to delineate the molecular mechanism of transcellular Ca(2+) transport are seriously hampered by the lack of a suitable cell model. The present study describes the establishment and validation of a primary murine cell model of the distal convolution. Viable kidney tubules were isolated from mice expressing enhanced Green Fluorescent Protein (eGFP) under the control of a TRPV5 promoter (pTRPV5-eGFP), using Complex Object Parametric Analyser and Sorting (COPAS) technology. Tubules were grown into tight monolayers on semi-permeable supports. Radioactive (45)Ca(2+) assays showed apical-to-basolateral transport rates of 13.5 ± 1.2 nmol/h/cm(2), which were enhanced by the calciotropic hormones parathyroid hormone and 1,25-dihydroxy vitamin D3. Cell cultures lacking TRPV5, generated by crossbreeding pTRPV5-eGFP with TRPV5 knockout mice (TRPV5(-/-)), showed significantly reduced transepithelial Ca(2+) transport (26 % of control), for the first time directly confirming the key role of TRPV5. Most importantly, using this cell model, a novel molecular player in transepithelial Ca(2+) transport was identified: mRNA analysis revealed that ATP-dependent Ca(2+)-ATPase 4 (PMCA4) instead of PMCA1 was enriched in isolated tubules and downregulated in TRPV5(-/-) material. Immunohistochemical stainings confirmed co-localization of PMCA4 with TRPV5 in the distal convolution. In conclusion, a novel primary cell model with TRPV5-dependent Ca(2+) transport characteristics was successfully established, enabling comprehensive studies of transcellular Ca(2+) transport.

Regulation of the epithelial Ca²⁺ channel TRPV5 by reversible histidine phosphorylation mediated by NDPK-B and PHPT1

The kidney, together with bone and intestine, plays a crucial role in maintaining whole-body calcium (Ca(2+)) homoeostasis, which is primarily mediated by altering the reabsorption of Ca(2+) filtered by the glomerulus. The transient receptor potential-vanilloid-5 (TRPV5) channel protein forms a six- transmembrane Ca(2+)-permeable channel that regulates urinary Ca(2+) excretion by mediating active Ca(2+) reabsorption in the distal convoluted tubule of the kidney. Here we show that the histidine kinase, nucleoside diphosphate kinase B (NDPK-B), activates TRPV5 channel activity and Ca(2+) flux, and this activation requires histidine 711 in the carboxy-terminal tail of TRPV5. In addition, the histidine phosphatase, protein histidine phosphatase 1, inhibits NDPK-B-activated TRPV5 in inside/out patch experiments. This is physiologically relevant to Ca(2+) reabsorption in vivo, as short hairpin RNA knockdown of NDPK-B leads to decreased TRPV5 channel activity, and urinary Ca(2+) excretion is increased in NDPK-B(-/-) mice fed a high-Ca(2+) diet. Thus these findings identify a novel mechanism by which TRPV5 and Ca(2+) reabsorption is regulated by the kidney and support the idea that histidine phosphorylation plays other, yet-uncovered roles in mammalian biology.

TRPV5: a Ca(2+) channel for the fine-tuning of Ca(2+) reabsorption

TRPV5 is one of the two channels in the TRPV family that exhibit high selectivity to Ca(2+) ions. TRPV5 mediates Ca(2+) influx into cells as the first step to transport Ca(2+) across epithelia. The specialized distribution in the distal tubule of the kidney positions TRPV5 as a key player in Ca(2+) reabsorption. The responsiveness in expression and/or activity of TRPV5 to hormones such as 1,25-dihydroxyvitamin D3, parathyroid hormone, estrogen, and testosterone makes TRPV5 suitable for its role in the fine-tuning of Ca(2+) reabsorption. This role is further optimized by the modulation of TRPV5 trafficking and activity via its binding partners; co-expressed proteins; tubular factors such as calbindin-D28k, calmodulin, klotho, uromodulin, and plasmin; extracellular and intracellular factors such as proton, Mg(2+), Ca(2+), and phosphatidylinositol-4,5-bisphosphate; and fluid flow. These regulations allow TRPV5 to adjust its overall activity in response to the body's demand for Ca(2+) and to prevent kidney stone formation. A point mutation in mouse Trpv5 gene leads to hypercalciuria similar to Trpv5 knockout mice, suggesting a possible role of TRPV5 in hypercalciuric disorders in humans. In addition, the single nucleotide polymorphisms in Trpv5 gene prevalently present in African descents may contribute to the efficient renal Ca(2+) reabsorption among African descendants. TRPV5 represents a potential therapeutic target for disorders with altered Ca(2+) homeostasis.

Biological significance of calbindin-D9k within duodenal epithelium

Calbindin-D_9k (CaBP-9k) binds calcium with high affinity and regulates the distribution of free calcium in the cytoplasm. The expression of CaBP-9k is detected primarily in intestine that is vitamin D target tissue, and accumulates in the enterocytes of the duodenal villi. These enterocytes are the clearest example of vitamin D responsive cells, and the presence of CaBP-9k within them accentuates calcium absorption mediated by active transcellular calcium transport. It has been well established that the expression of CaBP-9k is mediated with vitamin D response element on its promoter and it regulates the amount of intracellular calcium in order to prevent cell death from reaching the toxicity of free calcium. There is now little doubt that glucocorticoid also decreases CaBP-9k expression in duodenal epithelial cells. In addition, it was reported that the level of CaBP-9k gene in enterocytes is increased in pregnancy when the plasma estradiol concentration is generally associated with a concomitant increase. Although calcium homeostasis was not disturbed in mice lacking the CaBP-9k gene, we found that CaBP-9k has a buffering role of free calcium in the cytosolic environment beyond that of calcium transfer. To expand our knowledge of the biological functions of CaBP-9k, our research has focused on defining the biological significance of intracellular CaBP-9k. Our findings suggest that the CaBP-9k gene is involved in compensatory induction of other calcium transporter genes in duodenal epithelial cells. This article summarizes the findings from recent studies on the expression and the functions of CaBP-9k in the small intestine.

TRP channels

TRP channels constitute a large superfamily of cation channel forming proteins, all related to the gene product of the transient receptor potential (trp) locus in Drosophila. In mammals, 28 different TRP channel genes have been identified, which exhibit a large variety of functional properties and play diverse cellular and physiological roles. In this article, we provide a brief and systematic summary of expression, function, and (patho)physiological role of the mammalian TRP channels.

Reduced parathyroid hormone-stimulated 1,25-dihydroxyvitamin d production in vitamin d sufficient postmenoposual women with low bone mass and idiopathic secondary hyperparathyroidism

OBJECTIVE

Distinguishing secondary hyperparathyroidism (sHPT) from eucalcemic primary hyperparathyroidism (EC-pHPT) is important. The objective of this study was to measure parathyroid hormone (PTH)-stimulated production of 1α,25-dihydroxyvitamin D (1,25[OH]2D) in early postmenopausal patients with idiopathic sHPT, who also fit the criteria for EC-pHPT, compared to age-matched controls.

METHODS

In this pilot case-control study, postmenopausal women aged 44 to 55 years with normal serum calcium (Ca), glomerular filtration rate (GFR) ≥65 mL/min, and 25-hydroxyvitamin D (25[OH]D) ≥75 nmol/L (30 ng/mL) were given an 8 hour infusion of PTH(1-34), 12 pmol/kg/h. Patients (n = 5) had elevated PTH, normal 1,25(OH)2D, and no hypercalciuria. Controls (n = 5) had normal PTH. At baseline, 4, and 8 hours, serum Ca, creatinine (Cr), phosphorus (P), 1,25(OH)2D, fibroblast growth factor (FGF23), and 24,25(OH)2D as well as urine Ca, P, Cr, and cAMP/GFR were measured. The fractional excretion of calcium (FeCa) and tubular reabsorption of phosphorus (TMP)/GFR were calculated.

RESULTS

Patients had lower 1,25(OH)2D levels (± SD) than controls at 4 (39.8 ± 6.9 versus 58.8 ± 6.7; P = .002) and 8 hours (56.4 ± 9.2 versus 105 ± 2.3; P = .003) of PTH infusion, attenuated after adjusting for higher body mass index (BMI) in patients (P = .05, .04), respectively. The 24,25(OH)2D levels were lower in patients than controls (1.9 ± 0.6 versus 3.4 ± 0.6, respectively; P = .007). No differences were seen in serum Ca or P, urine cAMP/GFR, TRP/GFR, FeCa, or PTH suppression at 8 hours (patients 50%, controls 64%).

CONCLUSION

Vitamin D sufficient patients who fit the criteria for EC-pHPT had reduced PTH-stimulated 1,25(OH)2D compared to controls, partially attributable to their higher BMI. Other causes of reduced 1,25(OH)2D production ruled out were excessive catabolism of vitamin D metabolites, elevated FGF23, and CYP27B1 mutation. Elevated BMI and idiopathic reduced PTH-stimulated 1,25(OH)2D production should be considered in the differential of sHPT.

The delicate balance between vitamin D, calcium and bone homeostasis: lessons learned from intestinal- and osteocyte-specific VDR null mice

The serum calcium levels and the calcium content of the skeleton are highly interdependent. Indeed, bone requires calcium to preserve its strength, but it is at the same time also the predominant calcium storage from which calcium can be mobilized to supply the serum pool. The active form of vitamin D [1,25(OH)2D] plays a crucial role in regulating the transfer of calcium between blood and bone, evidenced by experimental data obtained from systemic, intestinal-specific and osteocyte-specific vitamin D receptor (Vdr) null mice. In fact, 1,25(OH)2D is required to maintain normocalcemia and bone health by enhancing intestinal calcium absorption when dietary calcium intake is normal/low. When, however, insufficient calcium is absorbed via the intestine, 1,25(OH)2D levels will increase and will act on mature osteoblasts and osteocytes to minimize calcium levels in bone tissue in favor of the blood calcium pool. Mechanistically, the high 1,25(OH)2D levels enhance bone remodeling which leads to osteopenia, and suppress bone matrix mineralization by increasing the levels of mineralization inhibitors, which causes hyperosteoidosis and hypomineralization. Thus, depending on the intestinal calcium acquisition, 1,25(OH)2D will target the intestine and/or the skeleton to maintain calcium levels in serum within a normal range.

Cytochrome P450-mediated metabolism of vitamin D

The vitamin D signal transduction system involves a series of cytochrome P450-containing sterol hydroxylases to generate and degrade the active hormone, 1α,25-dihydroxyvitamin D3, which serves as a ligand for the vitamin D receptor-mediated transcriptional gene expression described in companion articles in this review series. This review updates our current knowledge of the specific anabolic cytochrome P450s involved in 25- and 1α-hydroxylation, as well as the catabolic cytochrome P450 involved in 24- and 23-hydroxylation steps, which are believed to initiate inactivation of the vitamin D molecule. We focus on the biochemical properties of these enzymes; key residues in their active sites derived from crystal structures and mutagenesis studies; the physiological roles of these enzymes as determined by animal knockout studies and human genetic diseases; and the regulation of these different cytochrome P450s by extracellular ions and peptide modulators. We highlight the importance of these cytochrome P450s in the pathogenesis of kidney disease, metabolic bone disease, and hyperproliferative diseases, such as psoriasis and cancer; as well as explore potential future developments in the field.

Vitamin D down-regulates TRPC6 expression in podocyte injury and proteinuric glomerular disease

The transient receptor potential cation channel C6 (TRPC6) is a slit diaphragm protein expressed by podocytes. TRPC6 gain-of-function mutations cause autosomal dominant focal segmental glomerulosclerosis. In acquired proteinuric renal disease, glomerular TRPC6 expression is increased. We previously demonstrated that acquired increased TRPC6 expression is ameliorated by antiproteinuric angiotensin receptor blockers and angiotensin-converting enzyme inhibitors. Vitamin D also has an antiproteinuric effect. We hypothesized that vitamin D reduces proteinuria by affecting TRPC6 expression in podocytes. Adriamycin-induced nephropathy increased TRPC6 mRNA and protein expression and induced proteinuria in rats. Treatment with 1,25-dihydroxyvitamin D3 (1,25-D3) normalized TRPC6 expression and reduced proteinuria. In vitro, podocyte injury induced by adriamycin exposure in cultured podocytes increased TRPC6 expression. Treatment of injured podocytes with 1,25-D3 dose dependently reduced adriamycin-induced TRPC6 expression. Chromatin immunoprecipitation analysis demonstrated that the vitamin D receptor directly binds to the TRPC6 promoter. Moreover, 1,25-D3 reduced TRPC6 promoter activity in a luciferase reporter assay. In 1,25-D3-deficient 25-hydroxy-1α-hydroxylase knockout mice, TRPC6 expression was increased, accompanied by podocyte foot process effacement and proteinuria. 1,25-D3 supplementation normalized TRPC6 expression, podocyte morphology, and proteinuria in these mice. These results demonstrate that vitamin D down-regulates the enhanced TRPC6 expression in in vivo and in vitro podocyte injury, possibly through a direct effect on TRPC6 promoter activity. This TRPC6 down-regulation could contribute to the antiproteinuric effect of vitamin D.

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.

Vitamin D and the kidney

The kidney is essential for the maintenance of normal calcium and phosphorus homeostasis. Calcium and inorganic phosphorus are filtered at the glomerulus, and are reabsorbed from tubular segments by transporters and channels which are regulated by 1α,25-dihydroxyvitamin (1α,25(OH)(2)D) and parathyroid hormone (PTH). The kidney is the major site of the synthesis of 1α,25(OH)(2)D under physiologic conditions, and is one of the sites of 24,25-dihydroxyvitamin D (24,25(OH)(2)D) synthesis. The activity of the 25(OH)D-1α-hydroxylase, the mixed function oxidase responsible for the synthesis of 1α,25(OH)(2)D, is regulated by PTH, 1α,25(OH)(2)D, fibroblast growth factor 23 (FGF23), inorganic phosphorus and other growth factors. Additionally, the vitamin D receptor which binds to, and mediates the activity of 1α,25(OH)(2)D, is widely distributed in the kidney. Thus, the kidney, by regulating multiple transport and synthetic processes is indispensible in the maintenance of mineral homeostasis in physiological states.

Vitamin D and diabetes

There is no doubt that vitamin D deficiency is the cause of several metabolic bone diseases, but vitamin D status is also linked to many major human diseases including immune disorders. Mounting data strengthen the link between vitamin D and diabetes, in particular T1D and T2D. Despite some inconsistencies between studies that associate serum 25(OH)D levels with the risk of developing T1D or T2D, there seems to be an overall trend for an inverse correlation between levels of 25(OH)D and both disorders. There is also compelling evidence that 1,25(OH)2D regulates b-cell function by different mechanisms, such as influencing insulin secretion by regulating intracellular levels of Ca2+, increasing β-cell resistance to apoptosis, and perhaps also increasing β-cell replication. The capacity of vitamin D, more specifically 1,25(OH)2D, to modulate immune responses is of particular interest for both the therapy and prevention of diabetes. In the case of T1D, vitamin D supplementation in prediabetic individuals could help prevent or reduce the initiation of autoimmune processes possibly by regulating thymic selection of the T-cell repertoire, decreasing the numbers of autoreactive T cells, and inducing Treg cells. Although immune modulation is generally discussed for the treatment of T1D, it is also relevant for T2D. Indeed, recent studies have shown that T2D patients have increased systemic inflammation and that this state can induce β-cell dysfunction and death. Supplementation trials with regular vitamin D for the protection against the development of T1D and T2D have generated some contradictory data, but many weaknesses can be identified in these trials as most were underpowered or open-labeled. However, the overwhelming strength of preclinical data and of the observational studies make vitamin D or its analogues strong candidates for the prevention or treatment of diabetes or its complications. However, proof of causality needs well-designed clinical trials and if positive, adequate dosing, regimen, and compound studies are needed to define the contribution of vitamin D status and therapy in the global diabetes problem. There are many confounding factors that need to be taken into consideration when translating successful vitamin D therapies in animal models into humans, for example, gender, age, lifestyle, and genetic background. To come to solid conclusions on the potential of vitamin D or its analogues in the prevention of or therapy for all forms of diabetes, it is clear that large prospective trials with carefully selected populations and end points will be needed, but should also receive high priority.

The role of vitamin D in pregnancy and lactation: insights from animal models and clinical studies

Maternal adaptations during pregnancy and lactation appear to provide calcium to fetus and neonate without relying on vitamin D or calcitriol. Consequently, the blood calcium, calciotropic hormones, and skeleton appear normal at birth in the offspring of mothers who are severely vitamin D deficient or who lack calcitriol or its receptor. It remains unclear whether skeletal or extraskeletal problems will develop postnatally from exposure to vitamin D deficiency in utero. During the neonatal period, calcitriol-stimulated intestinal calcium absorption becomes the dominant mechanism of calcium delivery. The vitamin D-deficient neonate is at risk to develop hypocalcemia, rickets, and possibly extraskeletal disorders (e.g., type 1 diabetes). Breastfed babies are at higher risk of vitamin D deficiency because normally little vitamin D or 25-hydroxyvitamin D passes into breast milk. Dosing recommendations during pregnancy and lactation should ensure that the baby is born vitamin D sufficient and maintained that way during infancy and beyond.

Membrane topology and intracellular processing of cyclin M2 (CNNM2)

Recently, mutations in the cyclin M2 (CNNM2) gene were identified to be causative for severe hypomagnesemia. In kidney, CNNM2 is a basolaterally expressed protein with predominant expression in the distal convoluted tubule. Transcellular magnesium (Mg(2+)) reabsorption in the distal convoluted tubule represents the final step before Mg(2+) is excreted into the urine, thus fine-tuning its final excretion via a tightly regulated mechanism. The present study aims to get insight in the structure of CNNM2 and to characterize its post-translational modifications. Here, membrane topology studies using intramolecular epitopes and immunocytochemistry showed that CNNM2 has an extracellular N terminus and an intracellular C terminus. This suggests that one of the predicted transmembrane regions might be re-entrant. By homology modeling, we demonstrated that the loss-of-function mutation as found in patients disturbs the potential ATP binding by the intracellular cystathionine β-synthase domains. In addition, the cellular processing pathway of CNNM2 was exposed in detail. In the endoplasmic reticulum, the signal peptidase complex cleaves off a large N-terminal signal peptide of about 64 amino acids. Mutagenesis screening showed that CNNM2 is glycosylated at residue Asn-112, stabilizing CNNM2 on the plasma membrane. Interestingly, co-immunoprecipitation studies evidenced that CNNM2a forms heterodimers with the smaller isoform CNNM2b. These new findings on CNNM2 structure and processing may aid to elucidate the physiological role of CNNM2 in Mg(2+) reabsorption in the kidney.

Signaling through the extracellular calcium-sensing receptor (CaSR)

The extracellular calcium ([Formula: see text])-sensing receptor (CaSR) was the first GPCR identified whose principal physiological ligand is an ion, namely extracellular Ca(2+). It maintains the near constancy of [Formula: see text] that complex organisms require to ensure normal cellular function. A wealth of information has accumulated over the past two decades about the CaSR's structure and function, its role in diseases and CaSR-based therapeutics. This review briefly describes the CaSR and key features of its structure and function, then discusses the extracellular signals modulating its activity, provides an overview of the intracellular signaling pathways that it controls, and, finally, briefly describes CaSR signaling both in tissues participating in [Formula: see text] homeostasis as well as those that do not. Factors controlling CaSR signaling include various factors affecting the expression of the CaSR gene as well as modulation of its trafficking to and from the cell surface. The dimeric cell surface CaSR, in turn, links to various heterotrimeric and small molecular weight G proteins to regulate intracellular second messengers, lipid kinases, various protein kinases, and transcription factors that are part of the machinery enabling the receptor to modulate the functions of the wide variety of cells in which it is expressed. CaSR signaling is impacted by its interactions with several binding partners in addition to signaling elements per se (i.e., G proteins), including filamin-A and caveolin-1. These latter two proteins act as scaffolds that bind signaling components and other key cellular elements (e.g., the cytoskeleton). Thus CaSR signaling likely does not take place randomly throughout the cell, but is compartmentalized and organized so as to facilitate the interaction of the receptor with its various signaling pathways.

Calcium and bone metabolism disorders during pregnancy and lactation

Pregnancy and lactation cause a substantial increase in demand for calcium that is met by different maternal adaptations within each period. Intestinal calcium absorption more than doubles during pregnancy, whereas the maternal skeleton resorbs to provide most of the calcium content of breast milk during lactation. These maternal adaptations also affect the presentation, diagnosis, and management of disorders of calcium and bone metabolism. Although some women may experience fragility fractures as a consequence of pregnancy or lactation, for most women, parity and lactation do not affect the long-term risks of low bone density, osteoporosis, or fracture.

Adenovirus-delivered microRNA targeting the vitamin D receptor reduces intracellular Ca²⁺ concentrations by regulating the expression of Ca²⁺-transport proteins in renal epithelial cells

What’s known on the subject? and What does the study add? Experimental data have shown that VDR overexpression in the duodenum and kidney cortex is a biological characteristic of genetic hypercalciuric stone-forming rats (GHS rat), and a link between idiopathic calcium stone formation and the microstatellite marker D12S339 (near the VDR locus) has been proven in humans. Our study shows that VDR can positively regulate the mRNA and protein expression of TRPV5, calbindin-D28k and PMCA1b in NRK cell lines. VDR knockdown results in a decrease in intracellular Ca²⁺ concentration in NRK cell lines. The effect of the elevated VDR level in the kidney on hypercalciuria and the underlying mechanisms need to be further addressed.

OBJECTIVE

• To determine the effects of vitamin D receptor (VDR) on hypercalciuria and the mechanisms underlying such effects.

MATERIALS AND METHODS

• The adenovirus vector-delivered microRNA targeting rat VDR was constructed. We infected the normal rat kidney epithelial cell line NRK (Cellbank, China) with the adenovirus and then collected the cells at 0, 48, 72, 96, 120 h after infection. The mRNA and protein levels of VDR and VDR-dependent epithelial Ca2+ transport proteins were detected using real-time polymerase chain reaction and Western blot assays, respectively. • Fluorescent Ca²⁺ indicator Fluo-4 NW (Fluo-4 NW calcium assay kit, Molecular Probes, Invitrogen, USA) and laser scanning confocal microscope (Olympus, FV500-IX71, Japan) were used to detect the cytosolic free Ca²⁺ concentration at different time points after infection.

RESULTS

• The mRNA and protein level of VDR, transient receptor potential vanilloid receptor subtype 5 (TRPV5), calbindin-D28k and plasma membrane Ca²⁺-ATPase (PMCA1b) in infected NRK cells was significantly lower at 72 and 96 h after infection than that in control cells. • There was no significant difference between the two groups in the mRNA and protein level of TRPV6 and the Na⁺/Ca²⁺-exchanger (NCX1). • Furthermore, VDR knockdown results in a decrease in intracellular Ca²⁺ concentration ([Ca²⁺]i) in NRK cell lines.

CONCLUSIONS

• Our study shows that VDR can positively regulate the mRNA and protein expression of TRPV5, calbindin-D28k and PMCA1b, but not of TRPV6 or NCX1, in NRK cell lines. VDR knockdown results in a decrease in [Ca²⁺]i in NRK cell lines. • The effect of the elevated VDR level in the kidney on hypercalciuria and the mechanisms underlying need to be further addressed.

Calcemic actions of vitamin D: effects on the intestine, kidney and bone

The analysis of mice that lack systemically the actions of the active form of vitamin D, 1,25(OH)₂D, has shown that 1,25(OH)₂D is an essential regulator of calcium homeostasis and that its actions are aimed at maintaining serum calcium levels within narrow limits. Especially the stimulation of intestinal calcium transport by 1,25(OH)₂D is important for calcium and bone homeostasis. The involved transporters are however still elusive. The targeted deletion of 1,25(OH)₂D action in chondrocytes has provided compelling evidence for a paracrine control of bone development and endocrine regulation of phosphate homeostasis by 1,25(OH)₂D. Targeting vitamin D receptor (VDR) function in other tissues will further enhance our understanding of the cell-type specific action of 1,25(OH)₂D. In this review, we will discuss the current understanding and remaining questions concerning the calcemic actions of 1,25(OH)₂D in the intestine, kidney and bone, with special focus on the evidence obtained by the use of transgenic mouse models.

Role of the calcium-sensing receptor in reducing the risk for calcium stones

The tight control of blood Ca2+ levels within a narrow range is essential for the performance of vital physiologic functions. Muscle contraction, neuronal excitation, and intracellular signaling processes acquisitively require Ca2+. It is the concerted action of intestine, bone, and kidney that controls the Ca2+ balance through the regulation of intestinal absorption, bone (de)mineralization, and renal excretion of Ca2+, respectively. Along the nephron, fine-tuning of blood Ca2+ levels takes place by Ca2+ reabsorption. The calciotropic hormones regulate Ca2+ transport processes, leading to whole-body Ca2+ homeostasis and, importantly, preserving a constant Ca2+ concentration in the blood. Defects in renal Ca2+ handling can lead to hypercalciuria, consecutive kidney stone formation, and obstructive nephropathy. Here we give an overview of the key players involved in normal Ca2+ management and describe the in-depth investigations on a renal hypercalciuric model of disease, the Trpv5 knockout mouse, which naturally displays molecular adaptations that prevent Ca2+ precipitation in the kidney.

SGK1-dependent stimulation of intestinal SGLT1 activity by vitamin D

The serum- and glucocorticoid-inducible kinase SGK1 has previously been shown to mediate the glucocorticoid-dependent stimulation of several intestinal transport systems including the electrogenic glucose transporter SGLT1. In squamous carcinoma cells, SGK1 expression is stimulated by 1,25(OH)₂D₃, the biologically active metabolite of vitamin D. The present study explored whether vitamin D influences the intestinal SGLT1 activity. Jejunal SGLT1 activity was determined by Ussing chamber experiments. Under a normal diet, the electrogenic glucose transport was similar in SGK1 knockout (sgk1 ( -/- )) and wild type mice (sgk1 ( +/+ )). Following a vitamin D-rich diet (14 days 10,000 I.U. vitamin D), the SGK1 transcript levels as well as the SGLT1 protein abundance were increased in sgk1(+/+) mice. Moreover, SGLT1 activity was increased in sgk1(+/+) mice but not in sgk1(-/-) mice following a vitamin D-rich diet. Furthermore, an oral glucose load was followed by an increase in the plasma glucose concentration to significantly higher values in sgk1(+/+) mice treated with a vitamin D-rich diet than in untreated sgk1(+/+) mice. In conclusion, vitamin D treatment upregulates the expression of SGK1, which in turn enhances SGLT1 activity.

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.

Intestinal Na(+)/Ca(2+) exchanger protein and gene expression are regulated by 1,25(OH)(2)D(3) in vitamin D-deficient chicks

The role of 1,25(OH)(2)D(3) on the intestinal NCX activity was studied in vitamin D-deficient chicks (-D) as well as the hormone effect on NCX1 protein and gene expression and the potential molecular mechanisms underlying the responses. Normal, -D and -D chicks treated with cholecalciferol or 1,25(OH)(2)D(3) were employed. In some experiments, -D chicks were injected with cycloheximide or with cycloheximide and 1,25(OH)(2)D(3) simultaneously. NCX activity was decreased by -D diet, returning to normal values after 50 IU daily of cholecalciferol/10 days or a dose of 1μg calcitriol/kg of b.w. for 15 h. Cycloheximide blocked NCX activity enhancement produced by 1,25(OH)(2)D(3). NCX1 protein and gene expression were diminished by -D diet and enhanced by 1,25(OH)(2)D(3). Vitamin D receptor expression was decreased by -D diet, effect that disappeared after 1,25(OH)(2)D(3) treatment. Rapid effects of 1,25(OH)(2)D(3) on intestinal NCX activity were also demonstrated. The abolition of the rapid effects through addition of Rp-cAMPS and staurosporine suggests that non genomic effects of 1,25(OH)(2)D(3) on NCX activity are mediated by activation of PKA and PKC pathways. In conclusion, 1,25(OH)(2)D(3) enhances the intestinal NCX activity in -D chicks through genomic and non genomic mechanisms.

Switch of PMCA4 splice variants in bovine epididymis results in altered isoform expression during functional sperm maturation

Ca(2+) and Ca(2+)-dependent signals are essential for sperm maturation and fertilization. In mouse sperm the plasma membrane Ca(2+)-ATPase (PMCA) isoform 4 plays a crucial role in Ca(2+) transport. The two major splice variants of PMCA4 are PMCA4a and PMCA4b. PMCA4a differs from PMCA4b in the mechanism of calmodulin binding and activation. PMCA4a shows a much higher basal activity and is more effective than PMCA4b in returning Ca(2+) to resting levels. Knock-out mice carrying a PMCA4-null mutation are infertile because their sperm cannot achieve a hyperactivated state of motility. As sperm reach functional maturity during their transit through the epididymis, the expression of PMCA4a and 4b was assessed in bull testis and epididymis. Quantitative PCR revealed that PMCA4b is the major splice variant in testis, caput, and corpus epididymidis. In contrast, PMCA4a is the major splice variant in cauda epididymidis, whereas sperm are transcriptionally silent. Immunohistochemical staining using a new antibody against bovine PMCA4a located the PMCA4a to the apical membrane of the epithelium of cauda epididymidis, whereas testis, caput, and corpus epididymidis were negative. Western blotting of testis, epididymis, and sperm isolated from caput and cauda epididymidis showed a much higher level of PMCA4a in cauda epididymidis and sperm from cauda epididymidis compared with testis membranes and sperm from caput epididymidis. These findings suggest that PMCA4a is transferred to bovine sperm membranes in cauda epididymidis. This isoform switch may facilitate a higher calcium turnover in sperm necessary to traverse the female genital tract.

TRPV5 and TRPV6 in transcellular Ca(2+) transport: regulation, gene duplication, and polymorphisms in African populations

TRPV5 and TRPV6 are unique members of the TRP super family. They are highly selective for Ca(2+) ions with multiple layers of Ca(2+)-dependent inactivation mechanisms, expressed at the apical membrane of Ca(2+) transporting epithelia, and robustly responsive to 1,25-dihydroxivitamin D(3). These features are well suited for their roles as Ca(2+) entry channels in the first step of transcellular Ca(2+) transport pathways, which are involved in intestinal absorption, renal reabsorption of Ca(2+), placental transfer of Ca(2+) to fetus, and many other processes. While TRPV6 is more broadly expressed in a variety of tissues such as esophagus, stomach, small intestine, colon, kidney, placenta, pancreas, prostate, uterus, salivary gland, and sweat gland, TRPV5 expression is relatively restricted to the distal convoluted tubule and connecting tubule of the kidney. There is only one TRPV6-like gene in fish and birds in comparison to both TRPV5 and TRPV6 genes in mammals, indicating TRPV5 gene was likely generated from duplication of TRPV6 gene during the evolution of mammals to meet the needs of complex renal function. TRPV5 and TRPV6 are subjected to vigorous regulations under physiological, pathological, and therapeutic conditions. The elevated TRPV6 level in malignant tumors such as prostate and breast cancers makes it a potential therapeutic target. TRPV6, and to a lesser extent TRPV5, exhibit unusually high levels of single nucleotide polymorphisms (SNPs) in African populations as compared to other populations, indicating TRPV6 gene was under selective pressure during or after humans migrated out of Africa. The SNPs of TRPV6 and TRPV5 likely contribute to the Ca(2+) conservation mechanisms in African populations.

Abnormal neurogenesis in the dentate gyrus of adult mice lacking 1,25-dihydroxy vitamin D3 (1,25-(OH)2 D3)

In this study, we employed 1α-hydroxylase knockout (1α-(OH)ase(-/-) ) mice to investigate the influence of 1,25-dihydroxy vitamin D(3) (1,25-(OH)(2) D(3) ) deficiency on the adult neurogenesis in the hippocampal dentate gyrus (DG). The numbers of both 24-hr-old BrdU(+) cells and proliferating cell nuclear antigen positive cells in 8-week-old 1α-(OH)ase(-/-) mice increased approximately twofold compared with wild-type littermates. In contrast, the numbers of 7- and 28-day-old BrdU(+) cells in 1α-(OH)ase(-/-) mice decreased by 50% compared with wild-type mice, while the proportion of BrdU(+) /NeuN(+) cells in BrdU(+) population showed no difference between 1α-(OH)ase(-/-) and wild-type mice. Apoptotic cells in the subgranular zone (SGZ) of DG markedly increased in 1α-(OH)ase(-/-) mice. Replenishment of 1,25-(OH)(2) D(3) , but not correction of serum calcium and phosphorus levels, completely prevented changes in the neurogenesis in 1α-(OH)ase(-/-) mice. The absence of 1,25-(OH)(2) D(3) led to an increase in the expression of L-type voltage-gated calcium channel (L-VGCC) and a decrease in the nerve growth factor (NGF) mRNA level. Treatment with the L-VGCC inhibitor nifedipine blocked the increased cell proliferations by 1,25-(OH)(2) D(3) deficiency. Administration of NGF significantly attenuated the loss of newborn neurons in 1α-(OH)ase(-/-) mice.

Gastrointestinal calcium absorption in sheep is mostly insensitive to an alimentary induced challenge of calcium homeostasis

For ruminants, marked differences to monogastric species have been described concerning the localisation and vitamin D sensitivity of gastrointestinal calcium absorption, particularly with respect to the forestomach compartment. Therefore, we investigated gastrointestinal calcium transport of sheep as influenced by a dietary calcium restriction and/or a supraphysiological dosage of exogenous calcitriol. Using the Ussing chamber technique, we determined calcium and mannitol flux rates to differentiate between para- and transcellular calcium transport in rumen, duodenum, jejunum and colon. Expression of epithelial calcium channels, calbindin-D(9K), and basolateral extrusion mechanisms was determined by quantitative RT-PCR and Western blot analysis. Active calcium transport could be demonstrated in jejunum and rumen. A significant stimulation of jejunal calcium absorption was only observed in animals treated with calcitriol. The alimentary calcium restriction alone did not result in significant effects indicating a less effective intestinal adaptation to alimentary calcium restriction than observed in monogastric animals. The observed ruminal calcium transport was not affected at all, neither by the diet nor the calcitriol treatment. Furthermore, no significant expression of epithelial calcium channels or calbindin-D(9K) could be detected in the rumen; therefore it is concluded that calcium transport in the forestomachs is probably mediated by a different, so far unknown mechanism.

The renal connecting tubule: Resolved and unresolved issues in Ca(2+) transport

The renal connecting tubule (CNT) localizes to the distal part of the nephron between the distal convoluted tubule and the collecting duct, and consists of two different cell types: segment-specific and intercalated cells. The former reabsorb water (H(2)O), sodium (Na(+)) and calcium (Ca(2+)) ions to the blood compartment, while secreting potassium ions (K(+)) into the pro-urine. The latter cells contribute to the renal control of the acid-base balance. Several factors and hormones tightly regulate these transport processes. Although the CNT reabsorbs only ∼15% of filtered Ca(2+) load, this segment is finally decisive for the amount of Ca(2+) that appears in the urine. Impaired Ca(2+) transport across CNT can provoke severe urinary Ca(2+) excretion, called hypercalciuria. This review mainly focuses on the activity, abundance and expression of the epithelial Ca(2+) channel named Transient Receptor Potential Vanilloid 5 (TRPV5) that is the gatekeeper of active Ca(2+) reabsorption in the CNT.

High dose vitamin D3 attenuates the hypocalciuric effect of thiazide in hypercalciuric rats

Thiazide is known to decrease urinary calcium excretion. We hypothesized that thiazide shows different hypocalciuric effects depending on the stimuli causing hypercalciuria. The hypocalciuric effect of hydrochlorothiazide (HCTZ) and the expression of transient receptor potential vanilloid 5 (TRPV5), calbindin-D(28K), and several sodium transporters were assessed in hypercalciuric rats induced by high calcium diet and vitamin D(3). Urine calcium excretion and the expression of transporters were measured from 4 groups of Sprague-Dawley rats; control, HCTZ, high calcium-vitamin D, and high calcium-vitamin D with HCTZ groups. HCTZ decreased urinary calcium excretion by 51.4% in the HCTZ group and only 15% in the high calcium-vitamin D with HCTZ group. TRPV5 protein abundance was not changed by HCTZ in the high calcium-vitamin D with HCTZ group compared to the high calcium-vitamin D group. Protein abundance of NHE3, SGLT1, and NKCC2 decreased in the hypercalciuric rats, and only SGLT1 protein abundance was increased by HCTZ in the hypercalciuric rats. The hypocalciuric effect of HCTZ is attenuated in high calcium and vitamin D-induced hypercalciuric rats. This attenuation seems to have resulted from the lack of HCTZ's effect on protein abundance of TRPV5 in severe hypercalciuric condition induced by high calcium and vitamin D.

Vitamin D metabolism and rickets in domestic animals: a review

Rickets and osteomalacia are increasing in prevalence in people because of cultural practices, breast-feeding, decreased sun exposure, and increased sunscreen usage. Several hereditary forms of rickets owing to either renal phosphate wasting or defects in vitamin D metabolism are also reported in people. Rickets is well recognized in domestic animals, but published reports are not always supported by microscopic findings, and diagnoses based on clinical signs and radiology are unreliable. Most cases in domestic animals are caused by dietary deficiency of either vitamin D or phosphorus, but occasional inherited forms are reported in pigs, sheep, cats, and dogs. There is variation between species in susceptibility to dietary vitamin D and phosphorus deficiency and in the ability to manufacture vitamin D in their skin. A number of mouse models have been discovered or created to study human skeletal diseases and skeletal homeostasis. With the discovery that vitamin D is involved in not only calcium and phosphorus homeostasis but also in the immune system and cancer, there is great potential for new and existing animal models to generate valuable information about vitamin D and its many functions. This review presents an overview of vitamin D metabolism and rickets in domestic and laboratory animals and makes comparisons where appropriate with the disease in humans.

Methionine sulfoxide reductase B1 (MsrB1) recovers TRPM6 channel activity during oxidative stress

Mg(2+) is an essential ion for many cellular processes, including protein synthesis, nucleic acid stability, and numerous enzymatic reactions. Mg(2+) homeostasis in mammals depends on the equilibrium between intestinal absorption, renal excretion, and exchange with bone. The transient receptor potential melastatin type 6 (TRPM6) is an epithelial Mg(2+) channel, which is abundantly expressed in the luminal membrane of the renal and intestinal cells. It functions as the gatekeeper of transepithelial Mg(2+) transport. Remarkably, TRPM6 combines a Mg(2+)-permeable channel with an alpha-kinase domain. Here, by the Ras recruitment system, we identified methionine sulfoxide reductase B1 (MsrB1) as an interacting protein of the TRPM6 alpha-kinase domain. Importantly, MsrB1 and TRPM6 are both present in the renal Mg(2+)-transporting distal convoluted tubules. MsrB1 has no effect on TRPM6 channel activity in the normoxic conditions. However, hydrogen peroxide (H(2)O(2)) decreased TRPM6 channel activity. Co-expression of MsrB1 with TRPM6 attenuated the inhibitory effect of H(2)O(2) (TRPM6, 67 +/- 5% of control; TRPM6 + MsrB1, 81 +/- 5% of control). Cell surface biotinylation assays showed that H(2)O(2) treatment does not affect the expression of TRPM6 at the plasma membrane. Next, mutation of Met(1755) to Ala in TRPM6 reduced the inhibitory effect of H(2)O(2) on TRPM6 channel activity (TRPM6 M1755A: 84 +/- 10% of control), thereby mimicking the action of MsrB1. Thus, these data suggest that MsrB1 recovers TRPM6 channel activity by reducing the oxidation of Met(1755) and could, thereby, function as a modulator of TRPM6 during oxidative stress.

Vitamin D and diabetes

Type 1 (T1D) and type 2 (T2D) diabetes are considered multifactorial diseases in which both genetic predisposition and environmental factors participate in their development. Many cellular, preclinical, and observational studies support a role for vitamin D in the pathogenesis of both types of diabetes including: (1) T1D and T2D patients have a higher incidence of hypovitaminosis D; (2) pancreatic tissue (more specifically the insulin-producing beta-cells) as well as numerous cell types of the immune system express the vitamin D receptor (VDR) and vitamin D-binding protein (DBP); and (3) some allelic variations in genes involved in vitamin D metabolism and VDR are associated with glucose (in)tolerance, insulin secretion, and sensitivity, as well as inflammation. Moreover, pharmacologic doses of 1,25-dihydroxyvitamin D (1,25(OH)(2)D), the active form of vitamin D, prevent insulitis and T1D in nonobese diabetic (NOD) mice and other models of T1D, possibly by immune modulation as well as by direct effects on beta-cell function. In T2D, vitamin D supplementation can increase insulin sensitivity and decrease inflammation. This article reviews the role of vitamin D in the pathogenesis of T1D and T2D, focusing on the therapeutic potential for vitamin D in the prevention/intervention of T1D and T2D as well as its complications.

Expression of epithelial calcium transport system in rat cochlea and vestibular labyrinth

BACKGROUND

The low luminal Ca2+ concentration of mammalian endolymph in the inner ear is required for normal hearing and balance. We recently reported the expression of mRNA for a Ca2+-absorptive transport system in primary cultures of semicircular canal duct (SCCD) epithelium.

RESULTS

We now identify this system in native vestibular and cochlear tissues by qRT-PCR, immunoblots and confocal immunolocalization. Transcripts were found and quantified for several isoforms of epithelial calcium channels (TRPV5, TRPV6), calcium buffer proteins (calbindin-D9K, calbindin-D28K), sodium-calcium exchangers (NCX1, NCX2, NCX3) and plasma membrane Ca2+-ATPase (PMCA1, PMCA2, PMCA3, and PMCA4) in native SCCD, cochlear lateral wall (LW) and stria vascularis (SV) of adult rat as well as Ca2+ channels in neonatal SCCD. All components were expressed except TRPV6 in SV and PMCA2 in SCCD. 1,25-(OH)2vitamin D3 (VitD) significantly up-regulated transcripts of TRPV5 in SCCD, calbindin-D9K in SCCD and LW, NCX2 in LW, while PMCA4 in SCCD and PMCA3 in LW were down-regulated. The expression of TRPV5 relative to TRPV6 was in the sequence SV > Neonatal SCCD > Adult SCCD > LW > primary culture SCCD. Expression of TRPV5 protein from primary culture of SCCD did not increase significantly when cells were incubated with VitD (1.2 times control; P > 0.05). Immunolocalization showed the distribution of TRPV5 and TRPV6. TRPV5 was found near the apical membrane of strial marginal cells and both TRPV5 and TRPV6 in outer and inner sulcus cells of the cochlea and in the SCCD of the vestibular system.

CONCLUSIONS

These findings demonstrate for the first time the expression of a complete Ca2+ absorptive system in native cochlear and vestibular tissues. Regulation by vitamin D remains equivocal since the results support the regulation of this system at the transcript level but evidence for control of the TRPV5 channel protein was lacking.

The role of transient receptor potential channels in kidney disease

The transient receptor potential (TRP) superfamily consists, in mammals, of six protein subfamilies, TRPC, TRPM, TRPV, TRPA, TRPML and TRPP. TRPs are cation channels involved in many physiological processes and in the pathogenesis of various disorders. In the kidney, TRP channels are expressed along the nephron, and a role for some of these channels in renal function has been proposed. TRPC3 is thought to facilitate calcium ion influx into the principal cells of the collecting duct in response to vasopressin. TRPM3 and TRPV4 might be osmosensors, whereas the TRPP1/TRPP2 complex could function as a mechanosensor in the cilia of renal epithelial cells. A number of kidney diseases have also been linked to dysfunctional activity of TRPs. TRPC6 dysfunction has been associated with the onset of focal segmental glomerosclerosis; TRPP2 dysfunction is linked to autosomal-dominant polycystic kidney disease, TRPM6 mutations underlie hypomagnesemia with secondary hypocalcemia, and TRPV1 dysfunction is implicated in renal hypertension. A link between TRPC1 dysfunction and diabetic nephropathy has also been suggested in an animal model. Animal studies have implicated a role for TRPV5 in idiopathic hypercalciuria and vitamin D-dependent rickets, although these observations have not been confirmed in patients. This Review focuses on the role of renal TRP channels in health and disease.

Endogenous expression of TRPV5 and TRPV6 calcium channels in human leukemia K562 cells

In blood cells, changes in intracellular Ca2+ concentration ([Ca2+]i) are associated with multiple cellular events, including activation of cellular kinases and phosphatases, degranulation, regulation of cytoskeleton binding proteins, transcriptional control, and modulation of surface receptors. Although there is no doubt as to the significance of Ca2+ signaling in blood cells, there is sparse knowledge about the molecular identities of the plasmalemmal Ca2+ permeable channels that control Ca2+ fluxes across the plasma membrane and mediate changes in [Ca2+]i in blood cells. Using RNA expression analysis, we have shown that human leukemia K562 cells endogenously coexpress transient receptor potential vanilloid channels type 5 (TRPV5) and type 6 (TRPV6) mRNAs. Moreover, we demonstrated that TRPV5 and TRPV6 channel proteins are present in both the total lysates and the crude membrane preparations from leukemia cells. Immunoprecipitation revealed that a physical interaction between TRPV5 and TRPV6 may take place. Single-channel patch-clamp experiments demonstrated the presence of inwardly rectifying monovalent currents that displayed kinetic characteristics of unitary TRPV5 and/or TRPV6 currents and were blocked by extracellular Ca2+ and ruthenium red. Taken together, our data strongly indicate that human myeloid leukemia cells coexpress functional TRPV5 and TRPV6 calcium channels that may interact with each other and contribute into intracellular Ca2+ signaling.

Regulation of the epithelial Mg2+ channel TRPM6 by estrogen and the associated repressor protein of estrogen receptor activity (REA)

The maintenance of the Mg(2+) balance of the body is essential for neuromuscular excitability, protein synthesis, nucleic acid stability, and numerous enzymatic systems. The Transient Receptor Potential Melastatin 6 (TRPM6) functions as the gatekeeper of transepithelial Mg(2+) transport. However, the molecular regulation of TRPM6 channel activity remains elusive. Here, we identified the repressor of estrogen receptor activity (REA) as an interacting protein of TRPM6 that binds to the 6(th), 7(th), and 8(th) beta-sheets in its alpha-kinase domain. Importantly, REA and TRPM6 are coexpressed in renal Mg(2+)-transporting distal convoluted tubules (DCT). We demonstrated that REA significantly inhibits TRPM6, but not its closest homologue TRPM7, channel activity. This inhibition occurs in a phosphorylation-dependent manner, since REA has no effect on the TRPM6 phosphotransferase-deficient mutant (K1804R), while it still binds to this mutant. Moreover, activation of protein kinase C by phorbol 12-myristate 13-acetate-PMA potentiated the inhibitory effect of REA on TRPM6 channel activity. Finally, we showed that the interaction between REA and TRPM6 is a dynamic process, as short-term 17beta-estradiol treatment disassociates the binding between these proteins. In agreement with this, 17beta-estradiol treatment significantly stimulates the TRPM6-mediated current in HEK293 cells. These results suggest a rapid pathway for the effect of estrogen on Mg(2+) homeostasis in addition to its transcriptional effect. Together, these data indicate that REA operates as a negative feedback modulator of TRPM6 in the regulation of active Mg(2+) (re)absorption and provides new insight into the molecular mechanism of renal transepithelial Mg(2+) transport.

A missense mutation in the Kv1.1 voltage-gated potassium channel-encoding gene KCNA1 is linked to human autosomal dominant hypomagnesemia

Primary hypomagnesemia is a heterogeneous group of disorders characterized by renal or intestinal magnesium (Mg2+) wasting, resulting in tetany, cardiac arrhythmias, and seizures. The kidney plays an essential role in maintaining blood Mg2+ levels, with a prominent function for the Mg2+-transporting channel transient receptor potential cation channel, subfamily M, member 6 (TRPM6) in the distal convoluted tubule (DCT). In the DCT, Mg2+ reabsorption is an active transport process primarily driven by the negative potential across the luminal membrane. Here, we studied a family with isolated autosomal dominant hypomagnesemia and used a positional cloning approach to identify an N255D mutation in KCNA1, a gene encoding the voltage-gated potassium (K+) channel Kv1.1. Kv1.1 was found to be expressed in the kidney, where it colocalized with TRPM6 along the luminal membrane of the DCT. Upon overexpression in a human kidney cell line, patch clamp analysis revealed that the KCNA1 N255D mutation resulted in a nonfunctional channel, with a dominant negative effect on wild-type Kv1.1 channel function. These data suggest that Kv1.1 is a renal K+ channel that establishes a favorable luminal membrane potential in DCT cells to control TRPM6-mediated Mg2+ reabsorption.

Fluorescence isolation of mouse late distal convoluted tubules and connecting tubules: effects of vasopressin and vitamin D3 on Ca2+ signaling

The renal late distal convoluted tubules and connecting tubules are sites for the fine regulation of Na+ and Ca2+ reabsorption. The role of these segments in Na+ and K+ homeostasis is possibly underestimated, as the tubules are technically difficult to isolate in sufficient numbers and purity for functional analysis. To overcome these difficulties, we have developed a transgenic mouse model expressing enhanced green fluorescent protein in late distal convoluted tubules and connecting tubules. Enhanced green fluorescent protein expression was driven by the promoter for the transient receptor potential subfamily V, member 5. Confocal fluorescence microscopy allowed detection of enhanced green fluorescent protein in living, isolated late distal convoluted tubules and connecting tubules and in the initial cortical collecting ducts. Enhanced green fluorescent protein expression was validated by double- and triple-fluorescence immunolabeling with specific tubule markers. Freshly isolated late distal convoluted tubules and connecting tubules increased their intracellular Ca2+ levels in response to the V2 receptor-specific agonist deamino-Cys,d-Arg8-vasopressin (2 × 10−10 M) after 1 min of superfusion. In addition, both late distal convoluted tubules and connecting tubules displayed a concentration-dependent intracellular Ca2+ response to 1α,25-dihydroxyvitamin D3 (range 10−10 to 10−8 M). This suggests that 1α,25-dihydroxyvitamin D3 can act through a nongenomic signaling pathway in these tubules. In conclusion, the transgenic mouse model, expressing enhanced green fluorescent protein, is suitable for rapid isolation of viable late distal convoluted tubules, connecting tubules, and initial cortical collecting ducts and provides an ideal tool for a more exhaustive functional characterization of these segments.

Vitamin D and human health: lessons from vitamin D receptor null mice

The vitamin D endocrine system is essential for calcium and bone homeostasis. The precise mode of action and the full spectrum of activities of the vitamin D hormone, 1,25-dihydroxyvitamin D [1,25-(OH)(2)D], can now be better evaluated by critical analysis of mice with engineered deletion of the vitamin D receptor (VDR). Absence of a functional VDR or the key activating enzyme, 25-OHD-1alpha-hydroxylase (CYP27B1), in mice creates a bone and growth plate phenotype that mimics humans with the same congenital disease or severe vitamin D deficiency. The intestine is the key target for the VDR because high calcium intake, or selective VDR rescue in the intestine, restores a normal bone and growth plate phenotype. The VDR is nearly ubiquitously expressed, and almost all cells respond to 1,25-(OH)(2)D exposure; about 3% of the mouse or human genome is regulated, directly and/or indirectly, by the vitamin D endocrine system, suggesting a more widespread function. VDR-deficient mice, but not vitamin D- or 1alpha-hydroxylase-deficient mice, and man develop total alopecia, indicating that the function of the VDR and its ligand is not fully overlapping. The immune system of VDR- or vitamin D-deficient mice is grossly normal but shows increased sensitivity to autoimmune diseases such as inflammatory bowel disease or type 1 diabetes after exposure to predisposing factors. VDR-deficient mice do not have a spontaneous increase in cancer but are more prone to oncogene- or chemocarcinogen-induced tumors. They also develop high renin hypertension, cardiac hypertrophy, and increased thrombogenicity. Vitamin D deficiency in humans is associated with increased prevalence of diseases, as predicted by the VDR null phenotype. Prospective vitamin D supplementation studies with multiple noncalcemic endpoints are needed to define the benefits of an optimal vitamin D status.

Role of calcium malabsorption in the development of secondary hyperparathyroidism after biliopancreatic diversion

Secondary hyperparathyroidism (SH) is a frequent metabolic complication of bariatric surgery. Around 70%of patients who undergo biliopancreatic diversion (BPD) have this complication in the long term. The aim of this study was to evaluate the relative influence of vitamin D deficiency and calcium malabsorption in the development of SH in patients who underwent BPD. We reviewed the mean values of PTH throughout the post-operative follow-up and of related biochemical data (25-hydroxyvitamin D, calcium, magnesium) of 121 patients who underwent BPD at our institute from November 1996 to November 2004 (mean follow-up 66 months). Mean PTH correlated negatively with mean 25-hydroxyvitamin D (r=-0.27, p=0.003) and with urinary calcium(r=-0.19, p=0.047), and positively with age (r=0.22, p=0.018). However, a high mean PTH was found in 48.7% patients with mean 25-hydroxyvitamin D >or=30 ng/ml and in 80.0% patients with mean 25-hydroxyvitamin D between 20 and 30 ng/ml. The mean PTH was normal in 5 patients without calcium supplements at present, and progressively increased in parallel to the calcium dose in the rest of patients, although mean 25-hydroxyvitamin D levels were not related to the calcium dose. Our data suggest that individual differences in active and/or passive calcium absorption determine intractable SH after BPD in around half of the patients who have normal levels of 25-hydroxyvitamin D and in 80% of patients with 25-hydroxyvitamin D levels between 20 and 30 ng/ml after BPD, worsening with age.

Development and progression of secondary hyperparathyroidism in chronic kidney disease: lessons from molecular genetics

The identification of the calcium-sensing receptor (CaSR) and the clarification of its role as the major regulator of parathyroid gland function have important implications for understanding the pathogenesis and evolution of secondary hyperthyroidism in chronic kidney disease (CKD). Signaling through the CaSR has direct effects on three discrete components of parathyroid gland function, which include parathyroid hormone (PTH) secretion, PTH synthesis, and parathyroid gland hyperplasia. Disturbances in calcium and vitamin D metabolism that arise owing to CKD diminish the level of activation of the CaSR, leading to increases in PTH secretion, PTH synthesis, and parathyroid gland hyperplasia. Each represents a physiological adaptive response by the parathyroid glands to maintain plasma calcium homeostasis. Studies of genetically modified mice indicate that signal transduction via the CaSR is a key determinant of parathyroid cell proliferation and parathyroid gland hyperplasia. Because enlargement of the parathyroid glands has important implications for disease progression and disease severity, it is possible that clinical management strategies that maintain adequate calcium-dependent signaling through the CaSR will ultimately prove useful in diminishing parathyroid gland hyperplasia and in modifying disease progression.

Enriched uranium affects the expression of vitamin D receptor and retinoid X receptor in rat kidney

An increasing awareness of the radiological impact of the nuclear power industry and other nuclear technologies is observed nowadays on general population. This led to renew interest to assess the health impact of the use of enriched uranium (EU). The aim of this work was to investigate in vivo the effects of a chronic exposure to EU on vitamin D(3) metabolism, a hormone essential in mineral and bone homeostasis. Rats were exposed to EU in their drinking water for 9 months at a concentration of 40 mg l(-1) (1mg/rat day). The contamination did not change vitamin D plasma level. Vitamin D receptor (vdr) and retinoid X receptor alpha (rxralpha), encoding nuclear receptors involved in the biological activities of vitamin D, showed a lower expression in kidney, while their protein levels were paradoxically increased. Gene expression of vitamin D target genes, epithelial Ca(2+) channel 1 (ecac1) and Calbindin-D28k (cabp-d28k), involved in renal calcium transport were decreased. Among the vitamin D target organs examined, these molecular modifications occurred exclusively in the kidney, which confirms that this organ is highly sensitive to uranium exposure. In conclusion, this study showed that a chronic exposure to EU affects both mRNA and protein expressions of renal nuclear receptors involved in vitamin D metabolism, without any modification of the circulating vitamin D.