Hypervitaminosis D Mediates Compensatory Ca2+Hyperabsorption in TRPV5 Knockout Mice

Confronting the global obesity epidemic: investigating the role and underlying mechanisms of vitamin D in metabolic syndrome management

The escalating prevalence of MetS, driven by global obesity trends, underscores the urgent need for innovative therapeutic strategies. To gain a deeper understanding of the therapeutic potential of vitamin D in addressing MetS, we embarked on a targeted literature review that thoroughly examines the scientific underpinnings and pivotal discoveries derived from pertinent studies, aiming to unravel the intricate mechanisms through which vitamin D exerts its effects on MetS and its components. This article explores the multifunctional role of vitamin D in the management of MetS, focusing on its regulatory effects on insulin sensitivity, lipid metabolism, inflammation, and immune response. Through an extensive review of current research, we unveil the complex mechanisms by which vitamin D influences MetS components, highlighting its potential as a therapeutic agent. Our analysis reveals that vitamin D's efficacy extends beyond bone health to include significant impacts on cellular and molecular pathways critical to MetS. We advocate for further research to optimize vitamin D supplementation as a component of precision medicine for MetS, considering the safety concerns related to dosage and long-term use.

Functional characterization of nutraceuticals using spectral clustering: Centrality of caveolae-mediated endocytosis for management of nitric oxide and vitamin D deficiencies and atherosclerosis

It is well recognized that redox imbalance, nitric oxide (NO), and vitamin D deficiencies increase risk of cardiovascular, metabolic, and infectious diseases. However, clinical studies assessing efficacy of NO and vitamin D supplementation have failed to produce unambiguous efficacy outcomes suggesting that the understanding of the pharmacologies involved is incomplete. This raises the need for using systems pharmacology tools to better understand cause-effect relationships at biological systems levels. We describe the use of spectral clustering methodology to analyze protein network interactions affected by a complex nutraceutical, Cardio Miracle (CM), that contains arginine, citrulline, vitamin D, and antioxidants. This examination revealed that interactions between protein networks affected by these substances modulate functions of a network of protein complexes regulating caveolae-mediated endocytosis (CME), TGF beta activity, vitamin D efficacy and host defense systems. Identification of this regulatory scheme and the working of embedded reciprocal feedback loops has significant implications for treatment of vitamin D deficiencies, atherosclerosis, metabolic and infectious diseases such as COVID-19.

Integrated View on the Role of Vitamin D Actions on Bone and Growth Plate Homeostasis

1,25(OH)₂D₃, the biologically active form of vitamin D₃, is a major regulator of mineral and bone homeostasis and exerts its actions through binding to the vitamin D receptor (VDR), a ligand‐activated transcription factor that can directly modulate gene expression in vitamin D‐target tissues such as the intestine, kidney, and bone. Inactivating VDR mutations or vitamin D deficiency during development results in rickets, hypocalcemia, secondary hyperparathyroidism, and hypophosphatemia, pointing to the critical role of 1,25(OH)₂D₃‐induced signaling in the maintenance of mineral homeostasis and skeletal health. 1,25(OH)₂D₃ is a potent stimulator of VDR‐mediated intestinal calcium absorption, thus increasing the availability of calcium required for proper bone mineralization. However, when intestinal calcium absorption is impaired, renal calcium reabsorption is increased and calcium is mobilized from the bone to preserve normocalcemia. Multiple cell types within bone express the VDR, thereby allowing 1,25(OH)₂D₃ to directly affect bone homeostasis. In this review, we will discuss different transgenic mouse models with either Vdr deletion or overexpression in chondrocytes, osteoblasts, osteocytes, or osteoclasts to delineate the direct effects of 1,25(OH)₂D₃ on bone homeostasis. We will address the bone cell type–specific effects of 1,25(OH)₂D₃ in conditions of a positive calcium balance, where the amount of (re)absorbed calcium equals or exceeds fecal and renal calcium losses, as well as during a negative calcium balance, due to selective Vdr knockdown in the intestine or triggered by a low calcium diet. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

1,25(OH)2 vitamin D3 stimulates active phosphate transport but not paracellular phosphate absorption in mouse intestine

KEY POINTS

Intestinal absorption of phosphate proceeds via an active/transcellular route mostly mediated by NaPi-IIb/Slc34a2 and a poorly characterized passive/paracellular pathway. Intestinal phosphate absorption and expression of NaPi-IIb are stimulated by 1,25(OH)₂ vitamin D₃ but whether NaPi-IIb is the only target under hormonal control remains unknown. We report that administration of 1,25(OH)₂ vitamin D₃ to wild-type mice resulted in the expected increase in active transport of phosphate in jejunum, without changing paracellular fluxes. Instead, the same treatment failed to alter phosphate transport in intestinal-depleted Slc34a2-deficient mice. In both genotypes, 1,25(OH)₂ vitamin D₃ induced similar hyperphosphaturic responses and changes in the plasma levels of FGF23 and PTH. While urinary phosphate loss induced by administration of 1,25(OH)₂ vitamin D₃ did not alter plasma phosphate, further studies should investigate whether chronic administration would lead to phosphate imbalance in mice with reduced active intestinal absorption.

ABSTRACT

Intestinal absorption of phosphate is stimulated by 1,25(OH)₂ vitamin D_3. At least two distinct mechanisms underlie phosphate absorption in the gut, an active transcellular transport requiring the Na⁺ /phosphate cotransporter NaPi-IIb/Slc34a2, and a poorly characterized paracellular passive pathway. 1,25(OH)₂ vitamin D₃ stimulates NaPi-IIb expression and function, and loss of NaPi-IIb reduces intestinal phosphate absorption. However, it is remains unknown whether NaPi-IIb is the only target for hormonal regulation by 1,25(OH)₂ vitamin D₃ . Here we compared the effects of intraperitoneal administration of 1,25(OH)₂ vitamin D₃ (2 days, once per day) in wild-type and intestinal-specific Slc34a2-deficient mice, and analysed trans- vs. paracellular routes of phosphate absorption. We found that treatment stimulated active transport of phosphate only in jejunum of wild-type mice, though NaPi-IIb protein expression was upregulated in jejunum and ileum. In contrast, 1,25(OH)₂ vitamin D₃ administration had no effect in Slc34a2-deficient mice, suggesting that the hormone specifically regulates NaPi-IIb expression. In both groups, 1,25(OH)₂ vitamin D₃ elicited the expected increase of plasma fibroblast growth factor 23 (FGF23) and reduction of parathyroid hormone (PTH). Treatment resulted in hyperphosphaturia (and hypercalciuria) in both genotypes, though mice remained normophosphataemic. While increased intestinal absorption and higher FGF23 can trigger the hyperphosphaturic response in wild types, only higher FGF23 can explain the renal response in Slc34a2-deficient mice. Thus, 1,25(OH)₂ vitamin D₃ stimulates intestinal phosphate absorption by acting on the active transcellular pathway mostly mediated by NaPi-IIb while the paracellular pathway appears not to be affected.

The use of LP533401 as a therapeutic option for renal osteodystrophy affects, renal calcium handling, vitamin D metabolism, and bone health in uremic rats

BACKGROUND

Klotho is a key regulator of phosphate and Ca²⁺-transport in the kidney. Recently, we showed that treatment with LP533401 improved bone health in rats with chronic kidney disease (CKD) via the normalization of serum phosphate resulting from the reduced renal expression of phosphate cotransporters, including Klotho.

METHODS

We evaluated the effect of LP533401 therapy on Klotho-expression-dependent Ca²⁺-transporters, renal calcium handling, and the potential consequences for the bone of uremic rats.

RESULTS

Treatment with LP533401 and its vehicle resulted in the inhibition of transient receptor potential vanilloid receptor subtypes 5 and 6 (TRPV5, TRPV6) and calbindin (CaBP-28k, CaBP-9k) expression. The compensatory acceleration in renal expression of Na+/Ca²⁺-exchanger, 25-hydroxyvitamin d-1α-hydroxylase (CYP27B1), the intensification of vitamin D metabolism, and disruption of sophisticated balance between 1,25-dihydroxyvitamin D-serotonin was observed, especially in rats treated with LP533401. The imbalance between 1,25-dihydroxyvitamin D-serotonin levels led to intensified bone remodeling and improvement in bone geometry, mineral status, and strength in animals treated with LP533401.

CONCLUSION

The modulation of circulating serotonin and its relation to other regulators of calcium handling can play an important role in calcium homeostasis and bone integrity in CKD rats treated with LP533401.

Structural basis of TRPV5 channel inhibition by econazole revealed by cryo-EM

The transient receptor potential vanilloid 5 (TRPV5) channel is a member of the transient receptor potential (TRP) channel family, which is highly selective for Ca2+, that is present primarily at the apical membrane of distal tubule epithelial cells in the kidney and plays a key role in Ca2+ reabsorption. Here we present the structure of the full-length rabbit TRPV5 channel as determined using cryo-EM in complex with its inhibitor econazole. This structure reveals that econazole resides in a hydrophobic pocket analogous to that occupied by phosphatidylinositides and vanilloids in TRPV1, thus suggesting conserved mechanisms for ligand recognition and lipid binding among TRPV channels. The econazole-bound TRPV5 structure adopts a closed conformation with a distinct lower gate that occludes Ca2+ permeation through the channel. Structural comparisons between TRPV5 and other TRPV channels, complemented with molecular dynamics (MD) simulations of the econazole-bound TRPV5 structure, allowed us to gain mechanistic insight into TRPV5 channel inhibition by small molecules.

Interspecies differences in PTH-mediated PKA phosphorylation of the epithelial calcium channel TRPV5

The epithelial calcium (Ca²⁺) channel TRPV5 (transient receptor potential vanilloid 5) is expressed in the distal convoluted tubule of the kidney and facilitates active Ca²⁺ reabsorption. This process is instrumental for the maintenance of Ca²⁺ homeostasis. Therefore, all aspects of TRPV5 function are tightly regulated by the calciotropic parathyroid hormone (PTH). Rabbit (rb)TRPV5 channel activity was shown to be stimulated upon PTH-mediated protein kinase A (PKA) phosphorylation. Since there is incomplete conservation of the PKA consensus motif (RR/QxT) across species, the aim of this study was to extend these findings to humans and characterize the expression and function of human (h)TRPV5. Functional differences between rbTRPV5 and hTRPV5 upon PTH stimulation were investigated using ⁴⁵Ca²⁺ uptake assays, Fura-2 Ca²⁺ imaging, and cell surface biotinylation. While PTH treatment enhanced rbTRPV5 channel activity, it did not stimulate hTRPV5 activity. Mutation of the human RQxT motif into rabbit RRxT (hTRPV5 Q706R) partially restored the sensitivity to PTH. An ancestral sequence reconstruction of TRPV5 orthologues demonstrated that the change in the RRxT motif coincides with the creation of another putative PKA motif (RGAS to RRAS) in the amino terminus of hTRPV5. Interestingly, a constitutively phosphorylated hTRPV5 mutant (hTRPV5 S141D) displayed significantly decreased channel function, while its plasma membrane abundance was increased. Taken together, PTH-mediated stimulation of TRPV5, via PKA, is not conserved in humans. Our data suggest that PTH regulation of TRPV5 is altered in humans, an important observation for future studies that may add to new concepts on the role of PTH in renal Ca²⁺ handling.

Lifelong challenge of calcium homeostasis in male mice lacking TRPV5 leads to changes in bone and calcium metabolism

Trpv5 plays an important role in calcium (Ca²⁺) homeostasis, among others by mediating renal calcium reabsorption. Accordingly, Trpv5 deficiency strongly stresses Ca²⁺ homeostasis in order to maintain stable serum Ca²⁺. We addressed the impact of lifelong challenge of calcium homeostasis on the bone phenotype of these mice.

Aging significantly increased serum 1,25(OH)₂D₃ and PTH levels in both genotypes but they were more elevated in Trpv5^−/− mice, whereas serum Ca²⁺ was not affected by age or genotype. Age-related changes in trabecular and cortical bone mass were accelerated in Trpv5^−/− mice, including reduced trabecular and cortical bone thickness as well as reduced bone mineralization. No effect of Trpv5 deficiency on bone strength was observed. In 78-week-old mice no differences were observed between the genotypes regarding urinary deoxypyridinoline, osteoclast number, differentiation and activity as well as osteoclast precursor numbers, as assessed by flow cytometry.

In conclusion, life-long challenge of Ca²⁺ homeostasis present in Trpv5^−/− mice causes accelerated bone aging and a low cortical and trabecular bone mass phenotype. The phenotype of the Trpv5^−/− mice suggests that maintenance of adequate circulatory Ca²⁺ levels in patients with disturbances in Ca²⁺ homeostasis should be a priority in order to prevent bone loss at older age.

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.

Urinary β-galactosidase stimulates Ca2+ transport by stabilizing TRPV5 at the plasma membrane

Transcellular Ca(2+)transport in the late distal convoluted tubule and connecting tubule (DCT2/CNT) of the kidney is a finely controlled process mediated by the transient receptor potential vanilloid type 5 (TRPV5) channel. A complex-type-N-glycan bound at the extracellular residue Asn358 of TRPV5 through post-translational glycosylation has been postulated to regulate the activity of TRPV5 channels. Using in vitro Ca(2+)transport assays, immunoblot analysis, immunohistochemistry, patch clamp electrophysiology and total internal reflection fluorescence microscopy, it is demonstrated that the glycosidase β-galactosidase (β-gal), an enzyme that hydrolyzes galactose, stimulates TRPV5 channel activity. However, the activity of the non-glycosylated TRPV(N358Q)mutant was not altered in the presence of β-gal, showing that the stimulation is dependent on the presence of the TRPV5N-glycan. In addition, β-gal was found to stimulate transcellular Ca(2+)transport in isolated mouse primary DCT2/CNT cells. β-gal expression was detected in the apical membrane of the proximal tubules, and the protein was found in mouse urine. In summary, β-gal is present in the pro-urine from where it is thought to stimulate TRPV5 activity.

Segmental transport of Ca²⁺ and Mg²⁺ along the gastrointestinal tract

Calcium (Ca(2+)) and magnesium (Mg(2+)) ions are involved in many vital physiological functions. Since dietary intake is the only source of minerals for the body, intestinal absorption is essential for normal homeostatic levels. The aim of this study was to characterize the absorption of Ca(2+) as well as Mg(2+) along the gastrointestinal tract at a molecular and functional level. In both humans and mice the Ca(2+) channel transient receptor potential vanilloid subtype 6 (TRPV6) is expressed in the proximal intestinal segments, whereas Mg(2+) channel transient receptor potential melastatin subtype 6 (TRPM6) is expressed in the distal parts of the intestine. A method was established to measure the rate of Mg(2+) absorption from the intestine in a time-dependent manner by use of (25)Mg(2+). In addition, local absorption of Ca(2+) and Mg(2+) in different segments of the intestine of mice was determined by using surgically implanted intestinal cannulas. By these methods, it was demonstrated that intestinal absorption of Mg(2+) is regulated by dietary needs in a vitamin D-independent manner. Also, it was shown that at low luminal concentrations, favoring transcellular absorption, Ca(2+) transport mainly takes place in the proximal segments of the intestine, whereas Mg(2+) absorption predominantly occurs in the distal part of the gastrointestinal tract. Vitamin D treatment of mice increased serum Mg(2+) levels and 24-h urinary Mg(2+) excretion, but not intestinal absorption of (25)Mg(2+). Segmental cannulation of the intestine and time-dependent absorption studies using (25)Mg(2+) provide new ways to study intestinal Mg(2+) absorption.

[Disorders of calcium metabolism]

The majority of clinical complaints derive from disorders of calcium metabolism and are associated with a wide variety of clinical symptoms caused by numerous diseases with entirely different types of pathophysiology. The prognosis varies from favorable to fatal depending on the pathophysiology of the underlying disorder of calcium metabolism; therefore, the diagnostic work-up aims to quickly identify the underlying disease causing the disturbance in calcium homeostasis. Every clinical situation with a diminished state of calcium absorption is treated with calcium and vitamin D in varying doses whereas every disorder with an increased calcium absorptive or resorptive state is treated with improved diuresis in addition to antiresorptive drugs, such as bisphosphonates. In many situations the management of a disturbed calcium balance requires an interdisciplinary approach in order to treat the underlying disease in parallel with correction of the calcium homeostasis.

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.

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.

Elucidation of the distal convoluted tubule transcriptome identifies new candidate genes involved in renal Mg2+ handling

The kidney plays a key role in the maintenance of Mg2+ homeostasis. Specifically, the distal convoluted tubule (DCT) is instrumental in the fine-tuning of renal Mg2+ handling. In recent years, hereditary Mg2+ transport disorders have helped to identify important players in DCT Mg2+ homeostasis. Nevertheless, several proteins involved in DCT-mediated Mg2+ reabsorption remain to be discovered, and a full expression profile of this complex nephron segment may facilitate the discovery of new Mg2+-related genes. Here, we report Mg2+-sensitive expression of the DCT transcriptome. To this end, transgenic mice expressing enhanced green fluorescent protein under a DCT-specific parvalbumin promoter were subjected to Mg2+-deficient or Mg2+-enriched diets. Subsequently, the Complex Object Parametric Analyzer and Sorter allowed, for the first time, isolation of enhanced green fluorescent protein-positive DCT cells. RNA extracts thereof were analyzed by DNA microarrays comparing high versus low Mg2+ to identify Mg2+ regulatory genes. Based on statistical significance and a fold change of at least 2, 46 genes showed differential expression. Several known magnesiotropic genes, such as transient receptor potential cation channel, subfamily M, member 6 ( Trpm6), and Parvalbumin, were upregulated under low dietary Mg2+. Moreover, new genes were identified that are potentially involved in renal Mg2+ handling. To confirm that the selected candidate genes were regulated by dietary Mg2+ availability, the expression levels of solute carrier family 41, member 3 ( Slc41a3), pterin-4 α-carbinolamine dehydratase/dimerization cofactor of hepatocyte nuclear factor-1α ( Pcbd1), TBC1 domain family, member 4 ( Tbc1d4), and uromodulin ( Umod) were determined by RT-PCR analysis. Indeed, all four genes show significant upregulation in the DCT of mice fed a Mg2+-deficient diet. By elucidating the Mg2+-sensitive DCT transcriptome, new candidate genes in renal Mg2+ handling have been identified.

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.

Post-translational loss of renal TRPV5 calcium channel expression, Ca(2+) wasting, and bone loss in experimental colitis

BACKGROUND & AIMS

Dysregulated Ca(2+) homeostasis likely contributes to the etiology of inflammatory bowel disease-associated loss of bone mineral density. Experimental colitis leads to decreased expression of Klotho, a protein that supports renal Ca(2+) reabsorption by stabilizing the transient receptor potential vanilloid 5 (TRPV5) channel on the apical membrane of distal tubule epithelial cells.

METHODS

Colitis was induced in mice via administration of 2,4,6-trinitrobenzenesulfonic acid (TNBS) or transfer of CD4(+)interleukin-10(-/-) and CD4(+), CD45RB(hi) T cells. We investigated changes in bone metabolism, renal processing of Ca(2+), and expression of TRPV5.

RESULTS

Mice with colitis had normal serum levels of Ca(2+) and parathormone. Computed tomography analysis showed a decreased density of cortical and trabecular bone, and there was biochemical evidence for reduced bone formation and increased bone resorption. Increased fractional urinary excretion of Ca(2+) was accompanied by reduced levels of TRPV5 protein in distal convoluted tubules, with a concomitant increase in TRPV5 sialylation. In mouse renal intermedullary collecting duct epithelial (mIMCD3) cells transduced with TRPV5 adenovirus, the inflammatory cytokines tumor necrosis factor, interferon-γ, and interleukin-1β reduced levels of TRPV5 on the cell surface, leading to its degradation. Cytomix induced interaction between TRPV5 and UBR4 (Ubiquitin recoginition 4), an E3 ubiquitin ligase; knockdown of UBR4 with small interfering RNAs prevented cytomix-induced degradation of TRPV5. The effects of cytokines on TRPV5 were not observed in cells stably transfected with membrane-bound Klotho; TRPV5 expression was preserved when colitis was induced with TNBS in transgenic mice that overexpressed Klotho or in mice with T-cell transfer colitis injected with soluble recombinant Klotho.

CONCLUSIONS

After induction of colitis in mice via TNBS administration or T-cell transfer, tumor necrosis factor and interferon-γ reduced the expression and activity of Klotho, which otherwise would protect TRPV5 from hypersialylation and cytokine-induced TRPV5 endocytosis, UBR4-dependent ubiquitination, degradation, and urinary wasting of Ca(2+).

Vitamin D in thyroid tumorigenesis and development

Besides its classical role in bone and calcium homeostasis, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), the active form of vitamin D, has many non-classical effects; antiproliferative, anti-apoptotic and prodifferentiating effects of 1,25(OH)2D3 have been described in several tumour types in preclinical models. This review focuses on the insights gained in the elucidation of the role of 1,25(OH)2D3 in the normal thyroid and in the pathogenesis, progression and treatment of thyroid cancer, the most common endocrine malignancy. An increasing amount of observations points towards a role for impaired 1,25(OH)2D3-VDR signalling in the occurrence and progression of thyroid cancer, and a potential for structural analogues in the multimodal treatment of dedifferentiated iodine-resistant thyroid cancer. A role for vitamin D in thyroid-related autoimmunity is less convincing and needs further study. Altered 1,25(OH)2D3-VDR signalling does not influence normal thyroid development nor thyrocyte function, but does affect C-cell function, at least in rodents. If these findings also apply to humans deserves further study.

Autosomal dominant hypercalciuria in a mouse model due to a mutation of the epithelial calcium channel, TRPV5

Hypercalciuria is a major cause of nephrolithiasis, and is a common and complex disorder involving genetic and environmental factors. Identification of genetic factors for monogenic forms of hypercalciuria is hampered by the limited availability of large families, and to facilitate such studies, we screened for hypercalciuria in mice from an N-ethyl-N-nitrosourea mutagenesis programme. We identified a mouse with autosomal dominant hypercalciuria (HCALC1). Linkage studies mapped the Hcalc1 locus to a 11.94 Mb region on chromosome 6 containing the transient receptor potential cation channel, subfamily V, members 5 (Trpv5) and 6 (Trpv6) genes. DNA sequence analysis of coding regions, intron-exon boundaries and promoters of Trpv5 and Trpv6 identified a novel T to C transition in codon 682 of TRPV5, mutating a conserved serine to a proline (S682P). Compared to wild-type littermates, heterozygous (Trpv5^682P/+) and homozygous (Trpv5^682P/682P) mutant mice had hypercalciuria, polyuria, hyperphosphaturia and a more acidic urine, and ∼10% of males developed tubulointerstitial nephritis. Trpv5^682P/682P mice also had normal plasma parathyroid hormone but increased 1,25-dihydroxyvitamin D₃ concentrations without increased bone resorption, consistent with a renal defect for the hypercalciuria. Expression of the S682P mutation in human embryonic kidney cells revealed that TRPV5-S682P-expressing cells had a lower baseline intracellular calcium concentration than wild-type TRPV5-expressing cells, suggesting an altered calcium permeability. Immunohistological studies revealed a selective decrease in TRPV5-expression from the renal distal convoluted tubules of Trpv5^682P/+ and Trpv5^682P/682P mice consistent with a trafficking defect. In addition, Trpv5^682P/682P mice had a reduction in renal expression of the intracellular calcium-binding protein, calbindin-D_28K, consistent with a specific defect in TRPV5-mediated renal calcium reabsorption. Thus, our findings indicate that the TRPV5 S682P mutant is functionally significant and study of HCALC1, a novel model for autosomal dominant hypercalciuria, may help further our understanding of renal calcium reabsorption and hypercalciuria.

Functional TRPV6 channels are crucial for transepithelial Ca2+ absorption

TRPV6 is considered the primary protein responsible for transcellular Ca2+ absorption. In vitro studies demonstrate that a negatively charged amino acid (D) within the putative pore region of mouse TRPV6 (position 541) is critical for Ca2+ permeation of the channel. To elucidate the role of TRPV6 in transepithelial Ca2+ transport in vivo, we functionally analyzed a TRPV6D541A/D541A knockin mouse model. After weaning, mice were fed a regular (1% wt/wt) or Ca2+-deficient (0.02% wt/wt) diet and housed in metabolic cages. Blood was sampled for Ca2+ measurements, and the expression of Ca2+ transport proteins was analyzed in kidney and duodenum. Intestinal 45Ca2+ uptake was measured in vivo by an absorption assay. Challenging the mice with the Ca2+-deficient diet resulted in hypocalcemia in wild-type and TRPV6D541A/D541A mice. On a low-Ca2+ diet both mouse strains displayed increased expression of intestinal TRPV6, calbindin-D(9K), and renal TRPV5. TRPV6D541A/D541A mice showed significantly impaired intestinal Ca2+ uptake compared with wild-type mice, and duodenal TRPV5 expression was increased in TRPV6D541A/D541A mice. On a normal diet, serum Ca2+ concentrations normalized in both mouse strains. Under these conditions, intestinal Ca2+ uptake was similar, and the expression levels of renal and intestinal Ca2+ transport proteins were not affected. We demonstrate that TRPV6D541A/D541A mice exhibit impaired transcellular Ca2+ absorption. Duodenal TRPV5 expression was increased in TRPV6D541A/D541A mice, albeit insufficient to correct for the diminished Ca2+ absorption. Under normal conditions, when passive Ca2+ transport is predominant, no differences between wild-type and TRPV6D541A/D541A mice were observed. Our results demonstrate a specific role for TRPV6 in transepithelial Ca2+ absorption.

Regulation of transport in the connecting tubule and cortical collecting duct

The central goal of this overview article is to summarize recent findings in renal epithelial transport,focusing chiefly on the connecting tubule (CNT) and the cortical collecting duct (CCD).Mammalian CCD and CNT are involved in fine-tuning of electrolyte and fluid balance through reabsorption and secretion. Specific transporters and channels mediate vectorial movements of water and solutes in these segments. Although only a small percent of the glomerular filtrate reaches the CNT and CCD, these segments are critical for water and electrolyte homeostasis since several hormones, for example, aldosterone and arginine vasopressin, exert their main effects in these nephron sites. Importantly, hormones regulate the function of the entire nephron and kidney by affecting channels and transporters in the CNT and CCD. Knowledge about the physiological and pathophysiological regulation of transport in the CNT and CCD and particular roles of specific channels/transporters has increased tremendously over the last two decades.Recent studies shed new light on several key questions concerning the regulation of renal transport.Precise distribution patterns of transport proteins in the CCD and CNT will be reviewed, and their physiological roles and mechanisms mediating ion transport in these segments will also be covered. Special emphasis will be given to pathophysiological conditions appearing as a result of abnormalities in renal transport in the CNT and CCD.

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.

Calcium and bone disease

Calcium transport and calcium signaling are of basic importance in bone cells. Bone is the major store of calcium and a key regulatory organ for calcium homeostasis. Bone, in major part, responds to calcium-dependent signals from the parathyroids and via vitamin D metabolites, although bone retains direct response to extracellular calcium if parathyroid regulation is lost. Improved understanding of calcium transporters and calcium-regulated cellular processes has resulted from analysis of genetic defects, including several defects with low or high bone mass. Osteoblasts deposit calcium by mechanisms including phosphate and calcium transport with alkalinization to absorb acid created by mineral deposition; cartilage calcium mineralization occurs by passive diffusion and phosphate production. Calcium mobilization by osteoclasts is mediated by acid secretion. Both bone forming and bone resorbing cells use calcium signals as regulators of differentiation and activity. This has been studied in more detail in osteoclasts, where both osteoclast differentiation and motility are regulated by calcium.

Advances in the understanding of mineral and bone metabolism in inflammatory bowel diseases

Chronic inflammatory disorders such as inflammatory bowel diseases (IBDs) affect bone metabolism and are frequently associated with the presence of osteopenia, osteoporosis, and increased risk of fractures. Although several mechanisms may contribute to skeletal abnormalities in IBD patients, inflammation and inflammatory mediators such as TNF, IL-1β, and IL-6 may be the most critical. It is not clear whether the changes in bone metabolism leading to decreased mineral density are the result of decreased bone formation, increased bone resorption, or both, with varying results reported in experimental models of IBD and in pediatric and adult IBD patients. New data, including our own, challenge the conventional views, and contributes to the unraveling of an increasingly complex network of interactions leading to the inflammation-associated bone loss. Since nutritional interventions (dietary calcium and vitamin D supplementation) are of limited efficacy in IBD patients, understanding the pathophysiology of osteopenia and osteoporosis in Crohn's disease and ulcerative colitis is critical for the correct choice of available treatments or the development of new targeted therapies. In this review, we discuss current concepts explaining the effects of inflammation, inflammatory mediators and their signaling effectors on calcium and phosphate homeostasis, osteoblast and osteoclast function, and the potential limitations of vitamin D used as an immunomodulator and anabolic hormone in IBD.

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.

The TRPV5 promoter as a tool for generation of transgenic mouse models

The transient receptor potential vanilloid 5 (TRPV5) is a Ca(2+) channel, which is expressed in renal late distal convoluted tubules (DCT2s) and connecting tubules (CNTs). These tubules play a major role in hormone controlled renal Ca(2+) reabsorption, and thereby in body Ca(2+) homeostasis, as well as urinary excretion of other electrolytes, including Na(+) and K(+). DCT2 and CNT are difficult to distinguish from the surrounding structures and thereby to study by direct functional methods. We developed a transgenic mouse model expressing enhanced green fluorescent protein (EGFP) driven by the TRPV5 promoter to identify these specific tubules. Expression of EGFP in the DCT2 and CNT allows the isolation of pure DCT2 and CNT populations for proteomic and physiological analyses. The TRPV5 promoter is also useful for generating conditional knockout mouse models in a cell-specific manner. TRPV5 promoter driven Cre recombinase expression will be useful for inducing DCT2 and CNT specific gene silencing of various channels, pumps, carriers, and receptors. In this chapter, we describe the strategy for developing transgenic mouse lines involving the TRPV5 promoter, provide a description of extensive validation of these mouse lines, and discuss possible uses and limitations.

trpm7 regulation of in vivo cation homeostasis and kidney function involves stanniocalcin 1 and fgf23

The transient receptor potential melastatin 7 (trpm7) channel kinase is a primary regulator of magnesium homeostasis in vitro. Here we show that trpm7 is an important regulator of cation homeostasis as well as kidney function in vivo. Using zebrafish trpm7 mutants, we show that early larvae exhibit reduced levels of both total magnesium and total calcium. Accompanying these deficits, we show that trpm7 mutants express higher levels of stanniocalcin 1 (stc1), a potent regulator of calcium homeostasis. Using transgenic overexpression and morpholino oligonucleotide knockdown, we demonstrate that stc1 modulates both calcium and magnesium levels in trpm7 mutants and in the wild type and that levels of these cations are restored to normal in trpm7 mutants when stc1 activity is blocked. Consistent with defects in both calcium and phosphate homeostasis, we further show that trpm7 mutants develop kidney stones by early larval stages and exhibit increased levels of the anti-hyperphosphatemic factor, fibroblast growth factor 23 (fgf23). Finally, we demonstrate that elevated fgf23 expression contributes to kidney stone formation by morpholino knockdown of fgf23 in trpm7 mutants. Together, these analyses reveal roles for trpm7 in regulating cation homeostasis and kidney function in vivo and implicate both stc1 and fgf23 in these processes.

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.

The relative influence of calcium intake and vitamin D status on serum parathyroid hormone and bone turnover biomarkers in a double-blind, placebo-controlled parallel group, longitudinal factorial design

BACKGROUND

Adequate calcium and vitamin D are needed to maintain calcium balance.

OBJECTIVE

Our objective was to examine the influence of calcium intake and vitamin D exposure separately and their interaction on biomarkers of calcium sufficiency.

DESIGN

Healthy men and women, age 20-80 yr, were randomly allocated to four groups: 1) double placebo, 2) calcium (1200 mg daily) plus placebo, 3) vitamin D(3) (100 microg) plus placebo, and 4) vitamin D(3) and calcium. Fasting serum and urine as well as serum and urine 2 h after a calcium load (600 mg of calcium carbonate) were obtained at baseline and 3 months.

RESULTS

Ninety-nine participants were randomized; 78 completed the study. Baseline demographics, protein intake and laboratory studies did not differ among the four groups. Study medication compliance was 90%. Fasting bone turnover markers declined after 3 months only in the two groups given calcium supplements and increased in the vitamin D(3) plus placebo calcium group. The calcium load resulted in a decrease in PTH and in bone turnover markers that did not differ among groups. Urinary calcium excretion increased in the combined group. Mean serum 25-hydroxyvitamin D increased from a baseline of 67 (18 sd) nmol/liter to 111 (30 sd) nmol/liter after vitamin D supplementation.

CONCLUSION

Increased habitual calcium intake lowered markers of bone turnover. Acute ingestion of a calcium load lowered PTH and bone turnover markers. Additional intake of 100 microg/d vitamin D(3) did not lower PTH or markers of bone turnover.

Klotho prevents renal calcium loss

Disturbed calcium (Ca(2+)) homeostasis, which is implicit to the aging phenotype of klotho-deficient mice, has been attributed to altered vitamin D metabolism, but alternative possibilities exist. We hypothesized that failed tubular Ca(2+) absorption is primary, which causes increased urinary Ca(2+) excretion, leading to elevated 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] and its sequelae. Here, we assessed intestinal Ca(2+) absorption, bone densitometry, renal Ca(2+) excretion, and renal morphology via energy-dispersive x-ray microanalysis in wild-type and klotho(-/-) mice. We observed elevated serum Ca(2+) and fractional excretion of Ca(2+) (FE(Ca)) in klotho(-/-) mice. Klotho(-/-) mice also showed intestinal Ca(2+) hyperabsorption, osteopenia, and renal precipitation of calcium-phosphate. Duodenal mRNA levels of transient receptor potential vanilloid 6 (TRPV6) and calbindin-D(9K) increased. In the kidney, klotho(-/-) mice exhibited increased expression of TRPV5 and decreased expression of the sodium/calcium exchanger (NCX1) and calbindin-D(28K), implying a failure to absorb Ca(2+) through the distal convoluted tubule/connecting tubule (DCT/CNT) via TRPV5. Gene and protein expression of the vitamin D receptor (VDR), 25-hydroxyvitamin D-1-alpha-hydroxylase (1alphaOHase), and calbindin-D(9K) excluded renal vitamin D resistance. By modulating the diet, we showed that the renal Ca(2+) wasting was not secondary to hypercalcemia and/or hypervitaminosis D. In summary, these findings illustrate a primary defect in tubular Ca(2+) handling that contributes to the precipitation of calcium-phosphate in DCT/CNT. This highlights the importance of klotho to the prevention of renal Ca(2+) loss, secondary hypervitaminosis D, osteopenia, and nephrocalcinosis.

The calcium-sensing receptor promotes urinary acidification to prevent nephrolithiasis

Hypercalciuria increases the risk for urolithiasis, but renal adaptive mechanisms reduce this risk. For example, transient receptor potential vanilloid 5 knockout (TPRV5(-/-)) mice lack kidney stones despite urinary calcium (Ca(2+)) wasting and hyperphosphaturia, perhaps as a result of their significant polyuria and urinary acidification. Here, we investigated the mechanisms linking hypercalciuria with these adaptive mechanisms. Exposure of dissected mouse outer medullary collecting ducts to high (5.0 mM) extracellular Ca(2+) stimulated H(+)-ATPase activity. In TRPV5(-/-) mice, activation of the renal Ca(2+)-sensing receptor promoted H(+)-ATPase-mediated H(+) excretion and downregulation of aquaporin 2, leading to urinary acidification and polyuria, respectively. Gene ablation of the collecting duct-specific B1 subunit of H(+)-ATPase in TRPV5(-/-) mice abolished the enhanced urinary acidification, which resulted in severe tubular precipitations of Ca(2+)-phosphate in the renal medulla. In conclusion, activation of Ca(2+)-sensing receptor by increased luminal Ca(2+) leads to urinary acidification and polyuria. These beneficial adaptations facilitate the excretion of large amounts of soluble Ca(2+), which is crucial to prevent the formation of kidney stones.

Ageing and vitamin D deficiency: effects on calcium homeostasis and considerations for vitamin D supplementation

Vitamin D is a fat-soluble, seco-steroid hormone. In man, the vitamin D receptor is expressed in almost all tissues, enabling effects in multiple systems of the human body. These effects can be endocrine, paracrine and autocrine. The present review summarises the effects of ageing on the vitamin D endocrine system and on Ca homeostasis. Furthermore, consequences for vitamin D supplementation are discussed.

TRPV5 gene polymorphisms in renal hypercalciuria

BACKGROUND

Kidney stone formation is a major socioeconomic problem in humans, involving pain, recurrent treatment and renal insufficiency. As most renal precipitates contain calcium as a major component, hypercalciuria is the main risk factor for renal stone formation. Different forms of hypercalciuria can be classified, which primarily arise from defects in the main organs involved in calcium homeostasis. A distinction can be made between renal, absorptive and resorptive hypercalciuria, originating from disturbed calcium handling in kidney, intestine and bone, respectively. A positive family history predisposes individuals to an increased risk of stone formation, which strongly indicates the involvement of genetic susceptibility factors. TRPV5 is the renal epithelial calcium channel that is the gatekeeper protein in active calcium reabsorption in the kidney. TRPV5 gene ablation in mice leads to severe hypercalciuria, implying that TRPV5 is an interesting candidate gene for renal hypercalciuria in humans. This study aims to identify and functionally characterize TRPV5 gene aberrations in patients with renal hypercalciuria.

METHODS

The TRPV5 coding region and intron-exon boundaries were screened for gene mutations in 20 subjects displaying renal hypercalciuria after which identified non-synonymous polymorphisms were functionally characterized by patch-clamp analysis. Wild-type and TRPV5 channels including polymorphisms were transiently expressed in human embryonic kidney (HEK) 293 cells and functionally characterized by path-clamp analysis.

RESULTS

Genotyping TRPV5 in renal hypercalciuria patients revealed three non-synonymous and five synonymous polymorphisms. Electrophysiological characterization of the TRPV5 mutants did not reveal significant functional changes compared to wild-type TRPV5 channel recordings.

CONCLUSIONS

In this specific patient cohort, our data do not support a primary role for TRPV5 in the pathogenesis of renal hypercalciuria. However, TRPV5 cannot be excluded as a candidate gene in hypercalciuria.

Bone resorption inhibitor alendronate normalizes the reduced bone thickness of TRPV5(-/-) mice

TRPV5 is a Ca(2+)-selective channel involved in transcellular Ca(2+) absorption expressed in kidney and in the ruffled border of osteoclasts. Studies in hypercalciuric TRPV5 knockout (TRPV5(-/-)) mice, which display significantly increased vitamin D levels, showed that TRPV5 ablation increases number and size of osteoclasts but impairs osteoclast-mediated bone resorption. The latter is not in line with the observed decreased bone thickness in TRPV5(-/-) mice. Bisphosphonates also inhibit osteoclast-mediated bone resorption. The aim of this study was to evaluate the effect of alendronate on the expression of the Ca(2+) transporters in bone, kidney, and duodenum and, importantly, the bone phenotype in TRPV5(-/-) mice. Wildtype (TRPV5(+/+)) and TRPV5(-/-) mice were treated during 10 wk with 2 mg/kg alendronate or vehicle weekly and housed in metabolic cages at the end of treatment. Urine and blood samples were taken for biochemical analysis, and duodenum, kidney, and femur were sampled. Expression of Ca(2+) transporters and osteoclast ruffled border transporters in bone and cultured osteoclasts was determined by QPCR analysis. Femurs were scanned using muCT, and resorption pit assays were performed in bone marrow cultures isolated from TRPV5(+/+) and TRPV5(-/-) mice. Alendronate treatment enhanced bone thickness in TRPV5(+/+) mice but also normalized the disturbed bone morphometry parameters in TRPV5(-/-) mice. Bone TRPV5 expression was specifically enhanced by alendronate, whereas the expression of Ca(2+) transporters in kidney and intestine was not altered. The expression of the osteoclast ruffled border membrane proteins chloride channel 7 (CLC-7) and the vacuolar H(+)-ATPase did not differ between both genotypes, but alendronate significantly enhanced the expression and PTH levels in TRPV5(-/-) mice. The expression of TRPV5, CLC-7, and H(+)-ATPase in osteoclast cultures was not affected by alendronate. The number of resorption pits was reduced in TRPV5(-/-) bone marrow cultures, but the response to vitamin D was similar to that in TRPV5(+/+) cultures. The alendronate-induced upregulation of TRPV5 in bone together with the decreased resorptive capacity of TRPV5(-/-) osteoclasts in vitro suggests that TRPV5 has an important role in osteoclast function. However, our data indicate that significant bone resorption still occurs in TRPV5(-/-) mice, because alendronate treatment normalized bone thickness in these mice. Thus, TRPV5(-/-) mice are able to rescue the resulting defect in osteoclast-mediated bone resorption, possibly mediated by the long-term hypervitaminosis D or other (non)hormonal compensatory mechanisms.

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.

Mechanisms and regulation of epithelial Ca2+ absorption in health and disease

Ca2+ is essential for numerous physiological functions in our bodies. Therefore, its homeostasis is finely maintained through the coordination of intestinal absorption, renal reabsorption, and bone resorption. The Ca2+-selective epithelial channels TRPV5 and TRPV6 have been identified, and their physiological roles have been revealed: TRPV5 is important in final renal Ca2+ reabsorption, and TRPV6 has a key role in intestinal Ca2+ absorption. The TRPV5 knockout mice exhibit renal leak hypercalciuria and accordingly upregulate their intestinal TRPV6 expression to compensate for their negative Ca2+ balance. In contrast, despite their severe negative Ca2+ balance, TRPV6-null mice do not display any compensatory mechanism, thus resulting in secondary hyperparathyroidism. These results indicate that the genes for TRPV5 and TRPV6 are differentially regulated in human diseases associated with disturbed Ca2+ balance such as hypercalciuria, osteoporosis, and vitamin D-resistant rickets.

Calcium and phosphate homeostasis: concerted interplay of new regulators

Calcium (Ca(2+)) and phosphate (P(i)) are essential to many vital physiological processes. Consequently the maintenance of Ca(2+) and P(i) homeostasis is essential to a healthy existence. This occurs through the concerted action of intestinal, renal, and skeletal regulatory mechanisms. Ca(2+) and P(i) handling by these organs is under tight hormonal control. Disturbances in their homeostasis have been linked to pathophysiological disorders including chronic renal insufficiency, kidney stone formation, and bone abnormalities. Importantly, the kidneys fine-tune the amount of Ca(2+) and P(i) retained in the body by altering their (re)absorption from the glomerular filtrate. The ion transport proteins involved in this process have been studied extensively. Recently, new key players have been identified in the regulation of the Ca(2+) and P(i) balance. Novel regulatory mechanisms and their implications were introduced for the antiaging hormone klotho and fibroblast growth factor member 23 (FGF23). Importantly, transgenic mouse models, exhibiting disturbances in Ca(2+) and P(i) balance, have been of great value in the elucidation of klotho and FGF23 functioning. This review highlights the current knowledge and ongoing research into Ca(2+) and P(i) homeostasis, emphasizing findings from several relevant knockout mouse models.

Transcellular calcium transport in mammary epithelial cells

The time-honored paradigm for mammary gland transepithelial calcium transport into milk is centered on the view that most, if not all, calcium enters milk through the secretory pathway, and no ionic calcium directly crosses the apical plasma membrane. Data from several recent studies all strongly suggest that most calcium, in fact, is extruded across the apical plasma membrane directly by the plasma membrane calcium-ATPase isoform 2 (PMCA2). In this review we break down transcellular calcium transport into the tasks of calcium entry, calcium sequestration and compartmentalization, and calcium extrusion. We compare and contrast the steps of calcium transport into milk by mammary epithelial cells, and the specific molecules that might perform these tasks, with well-characterized calcium transport mechanisms in other epithelia, such as the kidney, small intestine, and salivary gland. Finally, we suggest an updated model for calcium transport into milk that incorporates calcium transport across the apical plasma membrane.

Functional significance of channels and transporters expressed in the inner ear and kidney

A number of ion channels and transporters are expressed in both the inner ear and kidney. In the inner ear, K+cycling and endolymphatic K+, Na+, Ca2+, and pH homeostasis are critical for normal organ function. Ion channels and transporters involved in K+cycling include K+channels, Na+-2Cl−-K+cotransporter, Na+/K+-ATPase, Cl−channels, connexins, and K+/Cl−cotransporters. Furthermore, endolymphatic Na+and Ca2+homeostasis depends on Ca2+-ATPase, Ca2+channels, Na+channels, and a purinergic receptor channel. Endolymphatic pH homeostasis involves H+-ATPase and Cl−/HCO3−exchangers including pendrin. Defective connexins (GJB2 and GJB6), pendrin (SLC26A4), K+channels (KCNJ10, KCNQ1, KCNE1, and KCNMA1), Na+-2Cl−-K+cotransporter (SLC12A2), K+/Cl−cotransporters (KCC3 and KCC4), Cl−channels (BSND and CLCNKA + CLCNKB), and H+-ATPase (ATP6V1B1 and ATPV0A4) cause hearing loss. All these channels and transporters are also expressed in the kidney and support renal tubular transport or signaling. The hearing loss may thus be paralleled by various renal phenotypes including a subtle decrease of proximal Na+-coupled transport (KCNE1/KCNQ1), impaired K+secretion (KCNMA1), limited HCO3−elimination (SLC26A4), NaCl wasting (BSND and CLCNKB), renal tubular acidosis (ATP6V1B1, ATPV0A4, and KCC4), or impaired urinary concentration (CLCNKA). Thus, defects of channels and transporters expressed in the kidney and inner ear result in simultaneous dysfunctions of these seemingly unrelated organs.

Mineral metabolism and aging: the fibroblast growth factor 23 enigma

PURPOSE OF REVIEW

The regulation of phosphate homeostasis was thought to be passively mediated by the calciotrophic hormones parathyroid hormone and 1,25(OH)2D3. This article summarizes the emerging trends that show an active regulation of phosphate homeostasis by fibroblast growth factor 23 (FGF-23) - a process fairly independent of calcium homeostasis - and how altered mineral ion metabolism may affect the aging process.

RECENT FINDINGS

A major breakthrough in FGF-23 biology has been achieved by the demonstration of strikingly similar physical/biochemical phenotypes of Fgf-23(-/-) and klotho hypomorph mice, which eventually led to the identification of klotho as a cofactor in FGF-23 and its receptor interactions. Furthermore, FGF-23 has emerged as a counter regulator of the renal 1alpha(OH)ase and sodium-phosphate cotransporter activities to modulate phosphate homeostasis. Finally, studies point towards a role of dentine matrix protein 1 in affecting phosphate homeostasis, in coordination with FGF-23.

SUMMARY

Recent mouse genetic studies have broadened our understanding of biochemical/molecular pathways involved in phosphate homeostasis, and linked FGF-23 to such regulation. Understanding the molecular interactions of essential calcium and phosphate regulators will enhance our knowledge of the coordinated regulation of mineral ion metabolism, and will help to redefine the molecular pathology of age-associated lesions accompanied by abnormal mineral ion metabolism such as vascular calcifications and osteoporosis.

Characterizing early events associated with the activation of target genes by 1,25-dihydroxyvitamin D3 in mouse kidney and intestine in vivo

In this report, we explore the interaction of the vitamin D receptor (VDR) at regulatory sites within both the Cyp24a1 and the Trpv6 genes using chromatin immunoprecipitation techniques in a mouse model in vivo. We show that exogenous 1,25(OH)(2)D(3) induces rapid VDR and RXR (retinoid X receptor) binding to the Cyp24a1 gene in both the kidney and the intestine and to the Trpv6 gene in the intestine. Separate studies of Trpv6 in vitro suggest that VDR binding occurs directly to VDR response elements located -2 and -4 kb upstream of the TSS. VDR binding is dose-dependent, demonstrating EC(50) values that are comparable with those for the induction of both Cyp24a1 and Trpv6 mRNA. Importantly, interaction of the VDR with these targets results in rapid changes in histone 4 acetylation as well as the recruitment of RNA polymerase II. The presence of both VDR and RNA polymerase II at these sites declines between 3-6 h, whereas the changes observed in acetylation decrease more slowly. Finally, we show that whereas mediator protein 1 is recruited to the Cyp24a1 promoter in the intestine, this coactivator is apparently not required for Trpv6 activation. These studies provide the first evidence for 1,25(OH)(2)D(3)-induced VDR interaction at key target genes in vivo, revealing the consequences of that interaction on the Cyp24a1 and Trpv6 genes.

TRP channels in kidney disease

Mammalian TRP channel proteins form six-transmembrane cation-permeable channels that may be grouped into six subfamilies on the basis of amino acid sequence homology (TRPC, TRPV, TRPM, TRPA, TRPP, and TRPML). Recent studies of TRP channels indicate that they are involved in numerous fundamental cell functions and are considered to play an important role in the pathophysiology of many diseases. Many TRPs are expressed in kidney along different parts of the nephron and growing evidence suggest that these channels are involved in hereditary, as well as acquired kidney disorders. TRPC6, TRPM6, and TRPP2 have been implicated in hereditary focal segmental glomerulosclerosis (FSGS), hypomagnesemia with secondary hypocalcemia (HSH), and polycystic kidney disease (PKD), respectively. In addition, the highly Ca(2+)-selective channel, TRPV5, contributes to several acquired mineral (dys)regulation, such as diabetes mellitus (DM), acid-base disorders, diuretics, immunosuppressant agents, and vitamin D analogues-associated Ca(2+) imbalance whereas TRPV4 may function as an osmoreceptor in kidney and participate in the regulation of sodium and water balance. This review presents an overview of the current knowledge concerning the distribution of TRP channels in kidney and their possible roles in renal physiology and kidney diseases.