TRPV5 and TRPV6 in Ca(2+) (re)absorption: regulating Ca(2+) entry at the gate

Widespread transcriptomic alterations of transient receptor potential channel genes in cancer

Ion channels, in particular transient-receptor potential (TRP) channels, are essential genes that play important roles in many physiological processes. Emerging evidence has demonstrated that TRP genes are involved in a number of diseases, including various cancer types. However, we still lack knowledge about the expression alterations landscape of TRP genes across cancer types. In this review, we comprehensively reviewed and summarised the transcriptomes from more than 10 000 samples in 33 cancer types. We found that TRP genes were widespreadly transcriptomic dysregulated in cancer, which was associated with clinical survival of cancer patients. Perturbations of TRP genes were associated with a number of cancer pathways across cancer types. Moreover, we reviewed the functions of TRP family gene alterations in a number of diseases reported in recent studies. Taken together, our study comprehensively reviewed TRP genes with extensive transcriptomic alterations and their functions will directly contribute to cancer therapy and precision medicine.

Effect of Tempeh and Daidzein on Calcium Status, Calcium Transporters, and Bone Metabolism Biomarkers in Ovariectomized Rats

Menopause marks a critical life stage characterized by hormonal changes that significantly impact bone health, leading to a heightened susceptibility to bone fractures. This research seeks to elucidate the impact of daidzein and tempeh on calcium status, calcium transporters, and bone metabolism in an ovariectomized rat model. Forty female Wistar rats, aged 3 months, participated in a two-phase experiment. The initial phase involved inducing a calcium deficit, while the second phase comprised dietary interventions across five groups: Sham (S) and Ovariectomy (O) with a standard diet, O with bisphosphonate (OB), O with pure daidzein (OD), and O with tempeh (OT). Multiple parameters, encompassing calcium levels, calcium transporters, bone histopathology, and serum bone metabolism markers, were evaluated. The findings revealed that the OT group showcased heightened levels of bone turnover markers, such as pyridinoline, C-telopeptide of type I collagen, bone alkaline phosphatase, and procollagen type I N-terminal propeptide, in contrast to S and O groups, with statistical significance (p < 0.05). Histopathologically, both the OD and OT groups exhibited effects akin to the OB group, indicating a decrease in the surface area occupied by adipocytes in the femoral bone structure, although statistically non-equivalent, supporting the directionally similar trends. Although TRPV5 and TRPV6 mRNA expression levels in the jejunum and duodenum did not display statistically significant differences (p > 0.05), the OD and OT groups exhibited increased expression compared to the O group. We hypothesized that obtained results may be related to the effect of isoflavones on estrogen pathways because of their structurally similar to endogenous estrogen and weak estrogenic properties. In conclusion, the daily consumption of pure daidzein and tempeh could potentially improve and reinstate calcium status, calcium transport, and bone metabolism in ovariectomized rats. Additionally, isoflavone products demonstrate effects similar to bisphosphonate drugs on these parameters in ovariectomized rats.

Yak milk promotes renal calcium reabsorption in mice with osteoporosis via the regulation of TRPV5

The Ca2+-selective epithelial channel TRPV5 plays a significant role in renal calcium reabsorption and improving osteoporosis (OP). In this study, we investigated the mechanisms of yak milk on osteoporosis mice in TRPV5-mediated Ca2+ reabsorption in the kidney. We observed that treatment of OP mice with yak milk reconstructed bone homeostasis demonstrated by increasing the levels of OPG as well as decreasing the levels of TRAP and ALP in serum. Additionally, yak milk reduced the level of parathyroid hormone (PTH) and elevated 1,25-(OH)2D3 and calcitonin (CT), and inhibited the excretion of Ca/Cr and Pi/Cr in OP mice, which explained by regulating hormone levels and thus enhance the renal Ca2+ reabsorption. Further analysis exhibited that yak milk upregulated the expression of TRPV5 protein and mRNA as well as calbindin-D28k in OP mice kidneys. Overall, these outcomes demonstrate that yak milk enhances renal Ca2+ reabsorption through the TRPV5 pathway synergistically with calbindin-D28k, thus ameliorating OP mice. This provides a new perspective for yak milk as a nutritional supplement to prevent osteoporosis.

A Novel Anti-TRPV6 Antibody and Its Application in Cancer Diagnosis In Vitro

Though the first discovery of TRPV6 channel expression in various tissues took place in the early 2000s, reliable tools for its protein detection in various cells and tissues are still missing. Here we show the generation and validation of rabbit polyclonal anti-TRPV6 channel antibodies (rb79-82) against four epitopes of 15 amino acids. Among them, only one antibody, rb79, was capable of detecting the full-length glycosylated form of the TRPV6 channel at around 100 kDa. The generated antibody was shown to be suitable for all in vitro applications, such as immunoblotting, immunoprecipitation, immunocytochemistry, immunofluorescence, etc. One of the most important applications is immunohistochemistry using the paraffin-embedded sections from cancer resection specimens. Using prostate cancer resection specimens, we have confirmed the absence of the TRPV6 protein in both healthy and benign hyperplasia, as well as its expression and correlation to the prostate cancer grades. Thus, the generated rabbit polyclonal anti-TRPV6 channel antibody rb79 is suitable for all in vitro diagnostic applications and particularly for the diagnosis in clinics using paraffin-embedded sections from patients suffering from various diseases and disorders involving the TRPV6 channel.

Calcium selective channel TRPV6: Structure, function, and implications in health and disease

Calcium-selective channel TRPV6 (Transient Receptor Potential channel family, Vanilloid subfamily member 6) belongs to the TRP family of cation channels and plays critical roles in transcellular calcium (Ca2+) transport, reuptake of Ca2+ into cells, and maintaining a local low Ca2+ environment for certain biological processes. Recent crystal and cryo-electron microscopy-based structures of TRPV6 have revealed mechanistic insights on how the protein achieves Ca2+ selectivity, permeation, and inactivation by calmodulin. The TRPV6 protein is expressed in a range of epithelial tissues such as the intestine, kidney, placenta, epididymis, and exocrine glands such as the pancreas, prostate and salivary, sweat, and mammary glands. The TRPV6 gene is a direct transcriptional target of the active form of vitamin D and is efficiently regulated to meet the body's need for Ca2+ demand. In addition, TRPV6 is also regulated by the level of dietary Ca2+ and under physiological conditions such as pregnancy and lactation. Genetic models of loss of function in TRPV6 display hypercalciuria, decreased bone marrow density, deficient weight gain, reduced fertility, and in some cases alopecia. The models also reveal that the channel plays an indispensable role in maintaining maternal-fetal Ca2+ transport and low Ca2+ environment in the epididymal lumen that is critical for male fertility. Most recently, loss of function mutations in TRPV6 gene is linked to transient neonatal hyperparathyroidism and early onset chronic pancreatitis. TRPV6 is overexpressed in a wide range of human malignancies and its upregulation is strongly correlated to tumor aggressiveness, metastasis, and poor survival in selected cancers. This review summarizes the current state of knowledge on the expression, structure, biophysical properties, function, polymorphisms, and regulation of TRPV6. The aberrant expression, polymorphisms, and dysfunction of this protein linked to human diseases are also discussed.

Klotho Level as a Marker of Low Bone Mineral Density in Egyptian Sickle Cell Disease Patients

Bone involvement of sickle cell disease (SCD) patients varies from acute clinical manifestations of painful vaso-occlusive crises or osteomyelitis to more chronic affection of bone mineral density (BMD) and debilitating osteonecrosis and osteoporosis. Secreted klotho protein is involved in calcium (Ca) reabsorption in the kidney. This study aimed to measure serum klotho levels in children with SCD to determine the possibility of using it as a marker of low BMD in children with SCD in correlation with a dual-energy radiograph absorptiometry scan. This study included 60 sickle disease patients and 30 age-matched and sex-matched control participants without SCD. A highly statistically significant difference was found between patients with normal BMD and those with low BMD, with serum Ca and klotho levels being lower in the latter group. Klotho serum level correlated positively with both serum Ca and BMD. Serum klotho level showed 94.9% sensitivity and 95.2% specificity in the detection of low BMD. Both serum Ca and klotho serum levels may be useful markers for detection of low BMD related to SCD with high sensitivity and specificity; however, klotho may be a better indicator as it is less affected by the nutritional and endocrinal status of patients or by intake of Ca supplements.

Dynamic evolution of transient receptor potential vanilloid (TRPV) ion channel family with numerous gene duplications and losses

The transient receptor potential vanilloid (TRPV) ion channel family is involved in multiple sensory and physiological functions including thermosensing and temperature-dependent neuroendocrine regulation. The objective of the present study was to investigate the number, origin and evolution of TRPV genes in metazoans, with special focus on the impact of the vertebrate whole-genome duplications (WGD). Gene searches followed by phylogenetic and synteny analyses revealed multiple previously undescribed TRPV genes. The common ancestor of Cnidaria and Bilateria had three TRPV genes that became four in the deuterostome ancestor. Two of these were lost in the vertebrate ancestor. The remaining two genes gave rise to two TRPV subfamilies in vertebrates, consisting of subtypes 1, 2, 3, 4, 9 and 5, 6, 7, 8, respectively. This gene expansion resulted from the two basal vertebrate WGD events (1R and 2R) and three local duplications before the radiation of gnathostomes. TRPV1, 4 and 5 have been retained in all gnathostomes investigated, presumably reflecting important functions. TRPV7 and 8 have been lost independently in various lineages but are still retained in cyclostomes, actinistians (coelacanth), amphibians, prototherians and basal actinopterygians (Polypteridae). TRPV3 and 9 are present in extant elasmobranchs, while TRPV9 was lost in the osteichthyan ancestor and TRPV3 in the actinopterygian ancestor. The coelacanth has retained the ancestral osteichthyan repertoire of TRPV1, 3, 4, 5, 7 and 8. TRPV2 arose in the tetrapod ancestor. Duplications of TRPV5 occurred independently in various lineages, such as cyclostomes, chondrichthyans, anuran amphibians, sauropsids, mammals (where the duplicate is called TRPV6), and actinopterygians (Polypteridae and Esocidae). After the teleost-specific WGD (3R) only TRPV1 retained its duplicate, whereas TRPV4 and 5 remained as single genes. Both 3R-paralogs of TRPV1 were kept in some teleost species, while one paralog was lost in others. The salmonid-specific WGD (4R) duplicated TRPV1, 4, and 5 leading to six TRPV genes. The largest number was found in Xenopus tropicalis with no less than 15 TRPV genes. This study provides a comprehensive evolutionary scenario for the vertebrate TRPV family, revealing additional TRPV types and proposing a phylogeny-based classification of TRPV across metazoans.

New Approaches to Assess Mechanisms of Action of Selective Vitamin D Analogues

Recent studies of transcription have revealed an advanced set of overarching principles that govern vitamin D action on a genome-wide scale. These tenets of vitamin D transcription have emerged as a result of the application of now well-established techniques of chromatin immunoprecipitation coupled to next-generation DNA sequencing that have now been linked directly to CRISPR-Cas9 genomic editing in culture cells and in mouse tissues in vivo. Accordingly, these techniques have established that the vitamin D hormone modulates sets of cell-type specific genes via an initial action that involves rapid binding of the VDR-ligand complex to multiple enhancer elements at open chromatin sites that drive the expression of individual genes. Importantly, a sequential set of downstream events follows this initial binding that results in rapid histone acetylation at these sites, the recruitment of additional histone modifiers across the gene locus, and in many cases, the appearance of H3K36me3 and RNA polymerase II across gene bodies. The measured recruitment of these factors and/or activities and their presence at specific regions in the gene locus correlate with the emerging presence of cognate transcripts, thereby highlighting sequential molecular events that occur during activation of most genes both in vitro and in vivo. These features provide a novel approach to the study of vitamin D analogs and their actions in vivo and suggest that they can be used for synthetic compound evaluation and to select for novel tissue- and gene-specific features. This may be particularly useful for ligand activation of nuclear receptors given the targeting of these factors directly to genetic sites in the nucleus.

TRPV3 and TRPV4 as candidate proteins for intestinal ammonium absorption

AIM

Absorption of ammonia from the gut has consequences that range from encephalitis in hepatic disease to global climate change induced by nitrogenous excretions from livestock. Since patch clamp data show that certain members of the transient receptor potential (TRP) family are permeable to NH₄ ⁺ , participation in ammonium efflux was investigated.

METHODS

Digesta, mucosa and muscular samples from stomach, duodenum, jejunum, ileum, caecum and colon of pigs were analysed via colourimetry, qPCR, Western blot, immunohistochemistry and Ussing chambers.

RESULTS

qPCR data show high duodenal expression of TRPV6. TRPM6 was highest in jejunum and colon, with expression of TRPM7 ubiquitous. TRPM8 and TRPV1 were below detection. TRPV2 was highest in the jejunum but almost non-detectable in the colon. TRPV4 was ubiquitously expressed by mucosal and muscular layers. TRPV3 mRNA was only found in the mucosa of the caecum and colon, organs in which NH₄ ⁺ was highest (>7 mmol·L^-1 ). Immunohistochemically, an apical expression of TRPV3 and TRPV4 could be detected in all tissues, with effects of 2-APB and GSK106790A supporting functional expression. In symmetrical NaCl Ringer, removal of mucosal Ca²⁺ and Mg²⁺ increased colonic short circuit current (I_sc ) and conductance (G_t ) by 0.18 ± 0.06 µeq·cm^-2 ·h^-1 and 4.70 ± 0.85 mS·cm^-2 (P < .05, N/n = 4/17). Application of mucosal NH₄ Cl led to dose-dependent and divalent-sensitive increases in G_t and I_sc , with effects highest in the caecum and colon.

CONCLUSION

We propose that TRP channels contribute to the intestinal transport of ammonium, with TRPV3 and TRPV4 promising candidate proteins. Pharmacological regulation may be possible.

Intestinal Calcium Absorption

In this article, we focus on mammalian calcium absorption across the intestinal epithelium in normal physiology. Intestinal calcium transport is essential for supplying calcium for metabolism and bone mineralization. Dietary calcium is transported across the mucosal epithelia via saturable transcellular and nonsaturable paracellular pathways, both of which are under the regulation of 1,25-dihydroxyvitamin D₃ and several other endocrine and paracrine factors, such as parathyroid hormone, prolactin, 17β-estradiol, calcitonin, and fibroblast growth factor-23. Calcium absorption occurs in several segments of the small and large intestine with varying rates and capacities. Segmental heterogeneity also includes differential expression of calcium transporters/carriers (e.g., transient receptor potential cation channel and calbindin-D_9k ) and the presence of favorable factors (e.g., pH, luminal contents, and gut motility). Other proteins and transporters (e.g., plasma membrane vitamin D receptor and voltage-dependent calcium channels), as well as vesicular calcium transport that probably contributes to intestinal calcium absorption, are also discussed. © 2021 American Physiological Society. Compr Physiol 11:1-27, 2021.

Parallel evolution of placental calcium transfer in the lizard Mabuya and eutherian mammals

An exceptional case of parallel evolution between lizards and eutherian mammals occurs in the evolution of viviparity. In the lizard genus Mabuya, viviparity provided the environment for the evolution of yolk-reduced eggs and obligate placentotrophy. One major event that favored the evolution of placentation was the reduction of the eggshell. As with all oviparous reptiles, lizard embryos obtain calcium from both the eggshell and egg yolk. Therefore, the loss of the eggshell likely imposes a constraint for the conservation of the egg yolk, which can only be obviated by the evolution of alternative mechanisms for the transport of calcium directly from the mother. The molecular and cellular mechanisms employed to solve these constraints, in a lizard with only a rudimentary eggshell such as Mabuya, are poorly understood. Here, we used RT-qPCR on placental and uterine samples during different stages of gestation in Mabuya, and demonstrate that transcripts of the calcium transporters trpv6, cabp28k, cabp9k and pmca are expressed and gradually increase in abundance through pregnancy stages, reaching their maximum expression when bone mineralization occurs. Furthermore, CABP28K/9K proteins were studied by immunofluorescence, demonstrating expression in specific regions of the mature placenta. Our results indicate that the machinery for calcium transportation in the Mabuya placenta was co-opted from other tissues elsewhere in the vertebrate bodyplan. Thus, the calcium transportation machinery in the placenta of Mabuya evolved in parallel with the mammalian placenta by redeploying the expression of similar calcium transporter genes.

Steroids and TRP Channels: A Close Relationship

Transient receptor potential (TRP) channels are remarkable transmembrane protein complexes that are essential for the physiology of the tissues in which they are expressed. They function as non-selective cation channels allowing for the signal transduction of several chemical, physical and thermal stimuli and modifying cell function. These channels play pivotal roles in the nervous and reproductive systems, kidney, pancreas, lung, bone, intestine, among others. TRP channels are finely modulated by different mechanisms: regulation of their function and/or by control of their expression or cellular/subcellular localization. These mechanisms are subject to being affected by several endogenously-produced compounds, some of which are of a lipidic nature such as steroids. Fascinatingly, steroids and TRP channels closely interplay to modulate several physiological events. Certain TRP channels are affected by the typical genomic long-term effects of steroids but others are also targets for non-genomic actions of some steroids that act as direct ligands of these receptors, as will be reviewed here.

Atomistic Insights of Calmodulin Gating of Complete Ion Channels

Intracellular calcium is essential for many physiological processes, from neuronal signaling and exocytosis to muscle contraction and bone formation. Ca²⁺ signaling from the extracellular medium depends both on membrane potential, especially controlled by ion channels selective to K⁺, and direct permeation of this cation through specialized channels. Calmodulin (CaM), through direct binding to these proteins, participates in setting the membrane potential and the overall permeability to Ca²⁺. Over the past years many structures of complete channels in complex with CaM at near atomic resolution have been resolved. In combination with mutagenesis-function, structural information of individual domains and functional studies, different mechanisms employed by CaM to control channel gating are starting to be understood at atomic detail. Here, new insights regarding four types of tetrameric channels with six transmembrane (6TM) architecture, Eag1, SK2/SK4, TRPV5/TRPV6 and KCNQ1–5, and its regulation by CaM are described structurally. Different CaM regions, N-lobe, C-lobe and EF3/EF4-linker play prominent signaling roles in different complexes, emerging the realization of crucial non-canonical interactions between CaM and its target that are only evidenced in the full-channel structure. Different mechanisms to control gating are used, including direct and indirect mechanical actuation over the pore, allosteric control, indirect effect through lipid binding, as well as direct plugging of the pore. Although each CaM lobe engages through apparently similar alpha-helices, they do so using different docking strategies. We discuss how this allows selective action of drugs with great therapeutic potential.

Bioimaging of Pb and STIM1 in mice liver, kidney and brain using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and immunohistochemistry

Lead (Pb) pollution is one of the most serious environmental problems and has attracted worldwide attention. Pb causes hematological, central nervous system, as well as renal toxicity, and so on. Although many investigations about Pb in blood to evaluate pollution status and toxic effects have been reported, there are open question about biological behavior of Pb. In order to reveal any toxicological mechanisms or influences, we focused on the local distribution of Pb in mice organs. Lead acetate (100 mg/L and 1000 mg/L) in drinking water were given to the BALB/c mice (male, seven weeks of age, N = 24) for three weeks. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) analysis revealed a homogenous distribution of Pb in the liver and inhomogeneous distribution in the kidney and brain. The hippocampus, thalamus, and hypothalamus had higher concentrations than other areas such as the white matter. Surprisingly, in the kidney, Pb tended to accumulate in the medulla rather than the cortex, strongly suggesting that high sensitivity areas and high accumulation areas differ. Moreover, distribution of stromal interacting protein 1 (STIM1) which is candidate gene of Pb pathway to the cells was homogenous in the liver and kidney whereas inhomogeneous in the brain. In contrast to our hypothesis, interestingly, Pb exposure under the current condition did not induce mRNA expressions for any candidate channel or transporter genes. Thus, further study should be conducted to elucidate the local distribution of Pb and other toxic metals, and pathway that Pb takes to the cells.

Calcium-sensing receptor regulates intestinal dipeptide absorption via Ca2+ signaling and IKCa activation

Although absorption of di- and tripeptides into intestinal epithelial cells occurs via the peptide transporter 1 (PEPT1, also called solute carrier family 15 member 1 (SLC15A1)), the detailed regulatory mechanisms are not fully understood. We examined: (a) whether dipeptide absorption in villous enterocytes is associated with a rise in cytosolic Ca2+ ([Ca2+ ]cyt ), (b) whether the calcium sensing receptor (CaSR) is involved in dipeptide-elicited [Ca2+ ]cyt signaling, and (c) what potential consequences of [Ca2+ ]cyt signaling may enhance enterocyte dipeptide absorption. Dipeptide Gly-Sar and CaSR agonist spermine markedly raised [Ca2+ ]cyt in villous enterocytes, which was abolished by NPS-2143, a selective CaSR antagonist and U73122, an phospholipase C (PLC) inhibitor. Apical application of Gly-Sar induced a jejunal short-circuit current (Isc), which was reduced by NPS-2143. CaSR expression was identified in the lamina propria and on the basal enterocyte membrane of mouse jejunal mucosa in both WT and Slc15a1-/- animals, but Gly-Sar-induced [Ca2+ ]cyt signaling was significantly decreased in Slc15a1-/- villi. Clotrimazole and TRM-34, two selective blockers of the intermediate conductance Ca2+ -activated K+ channel (IKCa ), but not iberiotoxin, a selective blocker of the large-conductance K+ channel (BKCa ) and apamin, a selective blocker of the small-conductance K+ channel (SKCa ), significantly inhibited Gly-Sar-induced Isc in native tissues. We reveal a novel CaSR-PLC-Ca2+ -IKCa pathway in the regulation of small intestinal dipeptide absorption, which may be exploited as a target for future drug development in human nutritional disorders.

Transient Receptor Potential Channels and Metabolism

Transient receptor potential (TRP) channels are nonselective cationic channels, conserved among flies to humans. Most TRP channels have well known functions in chemosensation, thermosensation, and mechanosensation. In addition to being sensing environmental changes, many TRP channels are also internal sensors that help maintain homeostasis. Recent improvements to analytical methods for genomics and metabolomics allow us to investigate these channels in both mutant animals and humans. In this review, we discuss three aspects of TRP channels, which are their role in metabolism, their functional characteristics, and their role in metabolic syndrome. First, we introduce each TRP channel superfamily and their particular roles in metabolism. Second, we provide evidence for which metabolites TRP channels affect, such as lipids or glucose. Third, we discuss correlations between TRP channels and obesity, diabetes, and mucolipidosis. The cellular metabolism of TRP channels gives us possible therapeutic approaches for an effective prophylaxis of metabolic syndromes.

Transient Receptor Potential Channels and Calcium Signaling

Transient receptor potential (TRP) cation channels play diverse roles in cellular Ca²⁺ signaling. First, as Ca²⁺-permeable channels that respond to a variety of stimuli, TRP channels can directly initiate cellular Ca²⁺ signals. Second, as nonselective cation channels, TRP channel activation leads to membrane depolarization, influencing Ca²⁺ influx via voltage-gated and store-operated Ca²⁺ channels. Finally, Ca²⁺ modulates the activity of most TRP channels, allowing them to function as molecular effectors downstream of intracellular Ca²⁺ signals. Whereas the TRP channel field has long been devoid of detailed channel structures, recent advances, particularly in cryo-electron microscopy-based structural approaches, have yielded a flurry of TRP channel structures, including members from all seven subfamilies. These structures, in conjunction with mutagenesis-based functional approaches, provided important new insights into the mechanisms whereby TRP channels permeate and sense Ca²⁺ These insights will be highly instrumental in the rational design of novel treatments for the multitude of TRP channel-related diseases.

Lithocholic Acid Is a Vitamin D Receptor Ligand That Acts Preferentially in the Ileum

The vitamin D receptor (VDR) is a nuclear receptor that mediates the biological action of the active form of vitamin D, 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃], and regulates calcium and bone metabolism. Lithocholic acid (LCA), which is a secondary bile acid produced by intestinal bacteria, acts as an additional physiological VDR ligand. Despite recent progress, however, the physiological function of the LCA−VDR axis remains unclear. In this study, in order to elucidate the differences in VDR action induced by 1,25(OH)₂D₃ and LCA, we compared their effect on the VDR target gene induction in the intestine of mice. While the oral administration of 1,25(OH)₂D₃ induced the Cyp24a1 expression effectively in the duodenum and jejunum, the LCA increased target gene expression in the ileum as effectively as 1,25(OH)₂D₃. 1,25(OH)₂D₃, but not LCA, increased the expression of the calcium transporter gene Trpv6 in the upper intestine, and increased the plasma calcium levels. Although LCA could induce an ileal Cyp24a1 expression as well as 1,25(OH)₂D₃, the oral LCA administration was not effective in the VDR target gene induction in the kidney. No effect of LCA on the ileal Cyp24a1 expression was observed in the VDR-null mice. Thus, the results indicate that LCA is a selective VDR ligand acting in the lower intestine, particularly the ileum. LCA may be a signaling molecule, which links intestinal bacteria and host VDR function.

Ca2+ Regulation of TRP Ion Channels

Ca²⁺ signaling influences nearly every aspect of cellular life. Transient receptor potential (TRP) ion channels have emerged as cellular sensors for thermal, chemical and mechanical stimuli and are major contributors to Ca²⁺ signaling, playing an important role in diverse physiological and pathological processes. Notably, TRP ion channels are also one of the major downstream targets of Ca²⁺ signaling initiated either from TRP channels themselves or from various other sources, such as G-protein coupled receptors, giving rise to feedback regulation. TRP channels therefore function like integrators of Ca²⁺ signaling. A growing body of research has demonstrated different modes of Ca²⁺-dependent regulation of TRP ion channels and the underlying mechanisms. However, the precise actions of Ca²⁺ in the modulation of TRP ion channels remain elusive. Advances in Ca²⁺ regulation of TRP channels are critical to our understanding of the diversified functions of TRP channels and complex Ca²⁺ signaling.

Vitamin D receptor suppresses proliferation and metastasis in renal cell carcinoma cell lines via regulating the expression of the epithelial Ca2+ channel TRPV5

We previously demonstrated that transient receptor potential vanilloid subfamily 5 (TRPV5) expression was decreased in renal cell carcinoma (RCC) compared with that in normal kidney tissues, a finding that was correlated with vitamin D receptor (VDR) expression, but further investigations is warranted. The aim of this study was to elucidate whether VDR could regulate the expression of TRPV5 and affect proliferation and metastasis in RCC. In this study, we used lentivirus to conduct the model of VDR overexpression and knockdown caki-1 and 786-O RCC cell lines in vitro. The results demonstrated that VDR overexpression significantly inhibited RCC cells proliferation, migration and invasion, and promoted apoptosis by the MTT, transwell cell migration/invasion and flow cytometry assays, respectively. However, VDR knockdown in RCC cells had the opposite effect. The RNA-sequence assay, which was assessed in caki-1 cells after VDR overexpression and knockdown, also indicated that significantly differentially expressed genes were associated with cell apoptotic, differentiation, proliferation and migration. RT-PCR and western blot analysis showed that VDR knockdown increased TRPV5 expression and VDR overexpression decreased TRPV5 expression in caki-1 cells. Furthermore, knockdown of TRPV5 expression suppressed the VDR knockdown-induced change in the proliferation, migration and invasion in caki-1 cells. Taken together, these findings confirmed that VDR functions as a tumour suppressor in RCC cells and suppresses the proliferation, migration and invasion of RCC through regulating the expression of TRPV5.

Crosslink between calcium and sodium signalling

NEW FINDINGS

What is the topic of this review? This paper overviews the links between Ca2+ and Na+ signalling in various types of cells. What advances does it highlight? This paper highlights the general importance of ionic signalling and overviews the molecular mechanisms linking Na+ and Ca2+ dynamics. In particular, the narrative focuses on the molecular physiology of plasmalemmal and mitochondrial Na+ -Ca2+ exchangers and plasmalemmal transient receptor potential channels. Functional consequences of Ca2+ and Na+ signalling for co-ordination of neuronal activity with astroglial homeostatic pathways fundamental for synaptic transmission are discussed.

ABSTRACT

Transmembrane ionic gradients, which are an indispensable feature of life, are used for generation of cytosolic ionic signals that regulate a host of cellular functions. Intracellular signalling mediated by Ca2+ and Na+ is tightly linked through several molecular pathways that generate Ca2+ and Na+ fluxes and are in turn regulated by both ions. Transient receptor potential (TRP) channels bridge endoplasmic reticulum Ca2+ release with generation of Na+ and Ca2+ currents. The plasmalemmal Na+ -Ca2+ exchanger (NCX) flickers between forward and reverse mode to co-ordinate the influx and efflux of both ions with membrane polarization and cytosolic ion concentrations. The mitochondrial calcium uniporter channel (MCU) and mitochondrial Na+ -Ca2+ exchanger (NCLX) mediate Ca2+ entry into and release from this organelle and couple cytosolic Ca2+ and Na+ fluctuations with cellular energetics. Cellular Ca2+ and Na+ signalling controls numerous functional responses and, in the CNS, provides for fast regulation of astroglial homeostatic cascades that are crucial for maintenance of synaptic transmission.

TRP channels in calcium homeostasis: from hormonal control to structure-function relationship of TRPV5 and TRPV6

Maintaining plasma calcium levels within a narrow range is of vital importance for many physiological functions. Therefore, calcium transport processes in the intestine, bone and kidney are tightly regulated to fine-tune the rate of absorption, storage and excretion. The TRPV5 and TRPV6 calcium channels are viewed as the gatekeepers of epithelial calcium transport. Several calciotropic hormones control the channels at the level of transcription, membrane expression, and function. Recent technological advances have provided the first near-atomic resolution structural models of several TRPV channels, allowing insight into their architecture. While this field is still in its infancy, it has increased our understanding of molecular channel regulation and holds great promise for future structure-function studies of these ion channels. This review will summarize the mechanisms that control the systemic calcium balance, as well as extrapolate structural views to the molecular functioning of TRPV5/6 channels in epithelial calcium transport.

High fat diet enriched with saturated, but not monounsaturated fatty acids adversely affects femur, and both diets increase calcium absorption in older female mice

Diet induced obesity has been shown to reduce bone mineral density (BMD) and Ca absorption. However, previous experiments have not examined the effect of high fat diet (HFD) in the absence of obesity or addressed the type of dietary fatty acids. The primary objective of this study was to determine the effects of different types of high fat feeding, without obesity, on fractional calcium absorption (FCA) and bone health. It was hypothesized that dietary fat would increase FCA and reduce BMD. Mature 8-month-old female C57BL/6J mice were fed one of three diets: a HFD (45% fat) enriched either with monounsaturated fatty acids (MUFAs) or with saturated fatty acids (SFAs), and a normal fat diet (NFD; 10% fat). Food consumption was controlled to achieve a similar body weight gain in all groups. After 8wk, total body bone mineral content and BMD as well as femur total and cortical volumetric BMD were lower in SFA compared with NFD groups (P<.05). In contrast, femoral trabecular bone was not affected by the SFAs, whereas MUFAs increased trabecular volume fraction and thickness. The rise over time in FCA was greater in mice fed HFD than NFD and final FCA was higher with HFD (P<.05). Intestinal calbindin-D9k gene and hepatic cytochrome P450 2r1 protein levels were higher with the MUFA than the NFD diet (P<.05). In conclusion, HFDs elevated FCA overtime; however, an adverse effect of HFD on bone was only observed in the SFA group, while MUFAs show neutral or beneficial effects.

The Calcium-Sensing Receptor in Health and Disease

The extracellular calcium-sensing receptor (CaSR) is a unique G protein-coupled receptor (GPCR) activated by extracellular Ca²⁺ and by other physiological cations including Mg²⁺, amino acids, and polyamines. CaSR is the most important master controller of the extracellular Ca²⁺ homeostatic system being expressed at high levels in the parathyroid gland, kidney, gut and bone, where it regulates parathyroid hormone (PTH) secretion, vitamin D synthesis, and Ca²⁺ absorption and resorption, respectively. Gain and loss of function mutations in the CaSR are responsible for severe disturbances in extracellular Ca²⁺ metabolism. CaSR agonists (calcimimetics) and antagonists (calcilytics) are in use or under intense research for treatment of hyperparathyroidism secondary to kidney failure and hypocalcemia with hypercalciuria, respectively. Expression of the CaSR extends to other tissues and systems beyond the extracellular Ca²⁺ homeostatic system including the cardiovascular system, the airways, and the nervous system where it may play physiological functions yet to be fully understood. As a consequence, CaSR has been recently involved in different pathologies including uncontrolled blood pressure, vascular calcification, asthma, and Alzheimer's disease. Finally, the CaSR has been shown to play a critical role in cancer either contributing to bone metastasis and/or acting as a tumor suppressor in some forms of cancer (parathyroid cancer, colon cancer, and neuroblastoma) and as oncogene in others (breast and prostate cancers). Here we review the role of CaSR in health and disease in calciotropic tissues and others beyond the extracellular calcium homeostatic system.

Aqueous extract of pomegranate seed attenuates glucocorticoid-induced bone loss and hypercalciuria in mice: A comparative study with alendronate

The present study was performed in order to examine bone loss and calcium homeostasis in mice with glucocorticoid (GC)-induced osteoporosis (GIOP) following treatment with the aqueous extract of pomegranate seed (AE-PS). In addition, a comparative study with alendronate was performed. Biomarkers in the serum and the urine were measured. The tibias, kidney and duodenum were removed in order to measure the levels of bone calcium, protein expression as well as to perform histomorphological analysis of the bone. GC treatment facilitated the induction of hypercalciuria in the mice, and the AE-PS‑treated mice exhibited a greater increase in serum calcium and a decrease in urine calcium. The AE-PS reversed the deleterious effects on the trabecular bone induced by DXM and stimulated bone remodeling, including an increase in bone calcium and alkaline phosphatase‑b (ALP-b) and a decrease in a the critical bone resorption markers C-terminal telopeptide of type I collagen (CTX) and tartrate‑resistant acid phosphatase-5b (TRAP-5b). Hematoxylin and eosin (H&E) staining revealed the increased disconnections and separation between the growth plate and the trabecular bone network as well as the reduction in the trabecular bone mass of the primary and secondary spongiosa throughout the proximal metaphysis of the tibia in the DXM group. Moreover, the decreased protein expression of transient receptor potential vanilloid (TRPV)5, TRPV6 and calbindin‑D9k (CaBP‑9k) was reversed by the AE-PS or alendronate supplementation in the kidneys and the duodenum as well as plasma membrane Ca2+‑ATPase1 (PMCA1) expression in the kidneys of mice with GIOP. There was no marked difference in pharmacological effectiveness between alendronate and the AE-PS. Taken together, these findings suggest that the AE-PS may be an alternative therapy suitable for use in the management of secondary osteoporosis.

Molecular Modeling of the Structural and Dynamical Changes in Calcium Channel TRPV5 Induced by the African-Specific A563T Variation

Transient receptor potential cation channels, vanilloid subfamily, member 5 (TRPV5) plays a key role in active Ca(2+) reabsorption in the kidney. Variations in TRPV5 occur at high frequency in African populations and may contribute to their higher efficiency of Ca(2+) reabsorption. One of the African specific variations, A563T, exhibits increased Ca(2+) transport ability. However, it is unclear how this variation influences the channel pore. On the basis of the structure of TRPV1, a TRPV5 model was generated to simulate the structural and dynamical changes induced by the A563T variation. On the basis of this model, amino acid residue 563 interacts with V540, which is one residue away from the key residue, D542, involved in Ca(2+) selectivity and Mg(2+) blockade. The A563T variation increases secondary structure stability and reduces dynamical motion of D542. In addition, the A563T variation alters the electrostatic potential of the outer surface of the pore. Differences in contact between selective filter residues and residue 563 and in electrostatic potential between the two TRPV5 variants were also observed in another model derived from an alternative alignment in the selective filters between TRPV5 and TRPV1. These findings indicate that the A563T variation induces structural, dynamical, and electrostatic changes in the TRPV5 pore, providing structural insight into the functional alterations associated with the A563T variation.

USP2-45 Is a Circadian Clock Output Effector Regulating Calcium Absorption at the Post-Translational Level

The mammalian circadian clock influences most aspects of physiology and behavior through the transcriptional control of a wide variety of genes, mostly in a tissue-specific manner. About 20 clock-controlled genes (CCGs) oscillate in virtually all mammalian tissues and are generally considered as core clock components. One of them is Ubiquitin-Specific Protease 2 (Usp2), whose status remains controversial, as it may be a cogwheel regulating the stability or activity of core cogwheels or an output effector. We report here that Usp2 is a clock output effector related to bodily Ca²⁺ homeostasis, a feature that is conserved across evolution. Drosophila with a whole-body knockdown of the orthologue of Usp2, CG14619 (dUsp2-kd), predominantly die during pupation but are rescued by dietary Ca²⁺ supplementation. Usp2-KO mice show hyperabsorption of dietary Ca²⁺ in small intestine, likely due to strong overexpression of the membrane scaffold protein NHERF4, a regulator of the Ca²⁺ channel TRPV6 mediating dietary Ca²⁺ uptake. In this tissue, USP2-45 is found in membrane fractions and negatively regulates NHERF4 protein abundance in a rhythmic manner at the protein level. In clock mutant animals (Cry1/Cry2-dKO), rhythmic USP2-45 expression is lost, as well as the one of NHERF4, confirming the inverse relationship between USP2-45 and NHERF4 protein levels. Finally, USP2-45 interacts in vitro with NHERF4 and endogenous Clathrin Heavy Chain. Taken together these data prompt us to define USP2-45 as the first clock output effector acting at the post-translational level at cell membranes and possibly regulating membrane permeability of Ca²⁺.

Deregulated Renal Calcium and Phosphate Transport during Experimental Kidney Failure

Impaired mineral homeostasis and inflammation are hallmarks of chronic kidney disease (CKD), yet the underlying mechanisms of electrolyte regulation during CKD are still unclear. Here, we applied two different murine models, partial nephrectomy and adenine-enriched dietary intervention, to induce kidney failure and to investigate the subsequent impact on systemic and local renal factors involved in Ca(2+) and Pi regulation. Our results demonstrated that both experimental models induce features of CKD, as reflected by uremia, and elevated renal neutrophil gelatinase-associated lipocalin (NGAL) expression. In our model kidney failure was associated with polyuria, hypercalcemia and elevated urinary Ca(2+) excretion. In accordance, CKD augmented systemic PTH and affected the FGF23-αklotho-vitamin-D axis by elevating circulatory FGF23 levels and reducing renal αklotho expression. Interestingly, renal FGF23 expression was also induced by inflammatory stimuli directly. Renal expression of Cyp27b1, but not Cyp24a1, and blood levels of 1,25-dihydroxy vitamin D3 were significantly elevated in both models. Furthermore, kidney failure was characterized by enhanced renal expression of the transient receptor potential cation channel subfamily V member 5 (TRPV5), calbindin-D28k, and sodium-dependent Pi transporter type 2b (NaPi2b), whereas the renal expression of sodium-dependent Pi transporter type 2a (NaPi2a) and type 3 (PIT2) were reduced. Together, our data indicates two different models of experimental kidney failure comparably associate with disturbed FGF23-αklotho-vitamin-D signalling and a deregulated electrolyte homeostasis. Moreover, this study identifies local tubular, possibly inflammation- or PTH- and/or FGF23-associated, adaptive mechanisms, impacting on Ca(2+)/Pi homeostasis, hence enabling new opportunities to target electrolyte disturbances that emerge as a consequence of CKD development.

A functional proteomics approach to the comprehension of sarcoidosis

Pulmonary sarcoidosis (Sar) is an idiopathic disease histologically typified by non-caseating epitheliod cell sarcoid granulomas. A cohort of 37 Sar patients with chronic persistent pulmonary disease was described in this study. BAL protein profiles from 9 of these Sar patients were compared with those from 8 smoker (SC) and 10 no-smoker controls (NSC) by proteomic approach. Principal Component Analysis was performed to clusterize the samples in the corresponding conditions highlighting a differential pattern profiles primarily in Sar than SC. Spot identification reveals thirty-four unique proteins involved in lipid, mineral, and vitamin Dmetabolism, and immuneregulation of macrophage function. Enrichment analysis has been elaborated by MetaCore, revealing 14-3-3ε, α1-antitrypsin, GSTP1, and ApoA1 as "central hubs". Process Network as well as Pathway Maps underline proteins involved in immune response and inflammation induced by complement system, innate inflammatory response and IL-6signalling. Disease Biomarker Network highlights Tuberculosis and COPD as pathologies that share biomarkers with sarcoidosis. In conclusion, Sar protein expression profile seems more similar to that of NSC than SC, conversely to other ILDs. Moreover, Disease Biomarker Network revealed several common features between Sar and TB, exhorting to orientate the future proteomics investigations also in comparative BALF analysis of Sar and TB.

Klotho, a new marker for osteoporosis and muscle strength in β-thalassemia major

Aim of this study was to compare plasma levels of the secreted protein Klotho in β-thalassemia major patients and in healthy controls. Also, we examined the existence of correlations between the protein level and osteoporosis, poor muscle strength and fractures. A total of 106 patients with β-thalassemia major and 95 healthy blood donors were enrolled. Klotho level in plasma was measured by mean of an ELISA test and the hand-grip strength using a dynamometer. Intact parathyroid hormone (PTH), 25-hydroxy vitamin D (Vitamin D), serum calcium (Ca), phosphate (P), total alkaline phosphatase (ALP), ferritin, creatinine were measured by standard clinical techniques. DXA was used to measure bone mineral density (BMD) at the lumbar spine (L2-L4), femoral neck and total hip. We found that the Klotho protein concentration was lower in the blood of patients with β-thalassemia major than in healthy controls, and it was directly correlated to the hand-grip strength. In β-thalassemia major patients, the secreted Klotho was lower than in healthy controls. The preliminary investigation into the correlation between markers of osteo- and sarcopenia and Klotho demonstrated a decreased Klotho concentration in β-TM patients and a higher probability of having had fragility fractures.

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.

The essence of female-male physiological dimorphism: differential Ca2+-homeostasis enabled by the interplay between farnesol-like endogenous sesquiterpenoids and sex-steroids? The Calcigender paradigm

Ca(2+) is the most omnipresent pollutant on earth, in higher concentrations a real threat to all living cells. When [Ca(2+)]i rises above 100 nM (=resting level), excess Ca(2+) needs to be confined in the SER and mitochondria, or extruded by the different Ca(2+)-ATPases. The evolutionary origin of eggs and sperm cells has a crucial, yet often overlooked link with Ca(2+)-homeostasis. Because there is no goal whatsoever in evolution, gametes did neither originate "with the purpose" of generating a progeny nor of increasing fitness by introducing meiosis. The explanation may simply be that females "invented the trick" to extrude eggs from their body as an escape strategy for getting rid of toxic excess Ca(2+) resulting from a sex-hormone driven increased influx into particular cells and tissues. The production of Ca(2+)-rich milk, seminal fluid in males and all secreted proteins by eukaryotic cells may be similarly explained. This view necessitates an upgrade of the role of the RER-Golgi system in extruding Ca(2+). In the context of insect metamorphosis, it has recently been (re)discovered that (some isoforms of) Ca(2+)-ATPases act as membrane receptors for some types of lipophilic ligands, in particular for endogenous farnesol-like sesquiterpenoids (FLS) and, perhaps, for some steroid hormones as well. A novel paradigm, tentatively named "Calcigender" emerges. Its essence is: gender-specific physiotypes ensue from differential Ca(2+)-homeostasis enabled by genetic differences, farnesol/FLS and sex hormones. Apparently the body of reproducing females gets temporarily more poisoned by Ca(2+) than the male one, a selective benefit rather than a disadvantage.

Protective effects of water fraction of Fructus Ligustri Lucidi extract against hypercalciuria and trabecular bone deterioration in experimentally type 1 diabetic mice

ETHNOPHARMACOLOGICAL RELEVANCE

Fructus Ligustri Lucidi (FLL), the fruit of Ligustrum lucidum Ait, is a commonly prescribed herb to nourish the endocrine and renal systems and to strengthen the bones in Traditional Chinese Medicine. This study was aimed to determine the effects of water fraction of FLL ethanol extract (WF-EE) on urinary calcium excretion and trabecular bone properties in type 1 diabetic mice.

MATERIALS AND METHODS

The DBA/2J mice with type 1 diabetes induced by streptozotocin injection were orally administered with WF-EE. After 6 weeks of treatment, the level of biomarkers, including serum calcium, parathyroid hormone (PTH), and fibroblast growth factor-23 (FGF-23) and urine calcium, was measured. Micro-CT was performed to detect trabecular bone properties of the proximal tibial metaphysis. The expression of active calcium transporting proteins in kidney and duodenum was determined by RT-PCR, immunoblotting and immunostaining.

RESULTS

Type 1 diabetes induced hypercalciuria and trabecular bone deterioration. The WF-EE could significantly inhibit hypercalciuria and ameliorate the micro-structure of trabecular bone as well as increase serum PTH and FGF-23 levels in type 1 diabetic mice. The gene expressions of active calcium transporting proteins in duodenum were up-regulated, and the gene and protein expressions of calcium-sensing receptor (CaSR) in kidney were dramatically down-regulated in diabetic mice in response to the treatment with WF-EE.

CONCLUSIONS

The present study demonstrated the protective effects of the water fraction of Fructus Ligustri Lucidi ethanol extract against hypercalciuria and trabecular bone deterioration in experimentally type 1 diabetic mice, and the underlying mechanism may be attributed to its regulations on duodenal calcium transporting proteins and renal CaSR.

The low PLC-δ1 expression in cystic fibrosis bronchial epithelial cells induces upregulation of TRPV6 channel activity

Increase of Ca(2+) influx in Cystic Fibrosis (CF) cells has been reported to be related to Transient Receptor Potential Canonical (TRPC6) channel, which is implicated in a functional coupling with Cystic Fibrosis Transmembrane conductance Regulator (CFTR). Several members of the Transient Receptor Potential Vanilloid (TRPV) channels family have already been described as emerging target for respiratory diseases. Two specific isoforms, TRPV5 and TRPV6 are of particular interest in the context of CF Ca(2+) homeostasis as they are highly selective toward Ca(2+) and constitutively activated. Thus, we investigated the involvement of these channels in Ca(2+) influx in CF and non-CF human bronchial epithelial cell lines. 16HBE14o-, CFBE41o- cell lines, primary human airway epithelial cells (hAEC) and freshly isolated human airway epithelial cells from CF and non-CF individuals were used. We showed that both channels are expressed in CF and non-CF cells and constitutive Ca(2+) influx was significantly higher (85%) in cells from CF individuals compared to cells from non-CF ones. Using the selective inhibitor of TRPV6 channel SOR-C27 and a siRNA strategy, our results revealed that TRPV6 was mostly involved in the increase of Ca(2+) influx. TRPV6 channel is negatively regulated by the PLC-PIP2 pathway. We measured the Ca(2+) influx in the presence of the non-specific PLC inhibitor, U73122, in non-CF human bronchial epithelial cells. Ca(2+) influx was increased by 33% with U73122 and this increase was largely reduced in the presence of SOR-C27. PLC inhibition in CF cells by U73122 had no effect on Ca(2+) influx. These results showed that PLC-PIP2 pathway is dysregulated in CF cells and leads to the increase of TRPV6 activity. The regulation of TRPV6 by PLC-PIP2 pathway implicates the specific PLC isoform, PLC-δ1. Immunoblot experiments revealed that expression of PLC-δ1 was decreased by 70% in CF cells. TRPV6 activity was normalized but not the level of expression of PLC-δ1 protein after F508del-CFTR rescue by low temperature for 48 h or treated for 24 h by 10 μM VX-809 in CF cells. This study revealed TRPV6 and PLC-δ1 as critical actor of Ca(2+) homeostasis in CF human bronchial epithelial cells.

TRPV6 calcium channel translocates to the plasma membrane via Orai1-mediated mechanism and controls cancer cell survival

Transient receptor potential vanilloid subfamily member 6 (TRPV6) is a highly selective calcium channel that has been considered as a part of store-operated calcium entry (SOCE). Despite its first discovery in the early 2000s, the role of this channel in prostate cancer (PCa) remained, until now, obscure. Here we show that TRPV6 mediates calcium entry, which is highly increased in PCa due to the remodeling mechanism involving the translocation of the TRPV6 channel to the plasma membrane via the Orai1/TRPC1-mediated Ca(2+)/Annexin I/S100A11 pathway, partially contributing to SOCE. The TRPV6 calcium channel is expressed de novo by the PCa cell to increase its survival by enhancing proliferation and conferring apoptosis resistance. Xenografts in nude mice and bone metastasis models confirmed the remarkable aggressiveness of TRPV6-overexpressing tumors. Immunohistochemical analysis of these demonstrated the increased expression of clinical markers such as Ki-67, prostate specific antigen, synaptophysin, CD31, and CD56, which are strongly associated with a poor prognosis. Thus, the TRPV6 channel acquires its oncogenic potential in PCa due to the remodeling mechanism via the Orai1-mediated Ca(2+)/Annexin I/S100A11 pathway.

First evidence of TRPV5 and TRPV6 channels in human parathyroid glands: possible involvement in neoplastic transformation

The parathyroid glands play an overall regulatory role in the systemic calcium (Ca²⁺) homeostasis. The purpose of the present study was to demonstrate the presence of the Ca²⁺ channels transient receptor potential vanilloid (TRPV) 5 and TRPV6 in human parathyroid glands. Semi-quantitative and quantitative PCR was carried out to evaluate the presence of TRPV5 and TRPV6 mRNAs in sporadic parathyroid adenomas and normal parathyroid glands. Western blot and immunocytochemical assays were used to assess protein expression, cellular localization and time expression in primary cultures from human parathyroid adenoma. TRPV5 and TRPV6 transcripts were then identified both in normal and pathological tissues. Predominant immunoreactive bands were detected at 75–80 kD for both vanilloid channels. These channels co-localized with the calcium-sensing receptor (CASR) on the membrane surface, but immunoreactivity was also detected in the cytosol and around the nuclei. Our data showed that western blotting recorded an increase of protein expression of both channels in adenoma samples compared with normal glands suggesting a potential relation with the cell calcium signalling pathway and the pathological processes of these glands.

Novel calcium ion channels TRPV5 and TRPV6 and gastrointestinal tumors

Transient receptor potential cation channel, subfamily V, member 5 (TRPV5) and TRPV6 are the subfamily members of the transient receptor potential (TRP), representing new highly selective Ca2+ membrane transport channels, which are mainly responsible for active transport of Ca2+ across the cell membrane and participate in regulation of many physiological activities in the body. This paper discusses the structures and electrophysiological properties of TRPV5 and TRPV6, their related factors and their relationship with gastrointestinal tumors, highlighting the role of TRPV5 and TRPV6 in the formation of gastrointestinal tumors.

Differences in gastrointestinal calcium absorption after the ingestion of calcium-free phosphate binders

Both calcium-containing and noncalcium-containing phosphate binders can increase gastrointestinal calcium absorption. Previously, we observed that lanthanum carbonate administration to rats with renal failure is not associated with increased calciuria. Additionally, lanthanum carbonate treatment in dialysis patients has been associated with a less pronounced initial decrease in serum parathyroid hormone compared with other phosphate binders. For 8 days, male Wistar rats received a diet supplemented with 2% lanthanum carbonate, 2% sevelamer, 2% calcium carbonate, or 2% cellulose. Calciuria was found to be increased in animals with normal renal function treated with sevelamer or calcium carbonate but not with lanthanum carbonate. In animals with renal failure, cumulative calcium excretion showed similar results. In rats with normal renal function, serum ionized calcium levels were increased after 2 days of treatment with sevelamer, while calcium carbonate showed a smaller increase. Lanthanum carbonate did not induce differences. In animals with renal failure, no differences were found between sevelamer-treated, calcium carbonate-treated, and control groups. Lanthanum carbonate, however, induced lower ionized calcium levels within 2 days of treatment. These results were confirmed in normal human volunteers, who showed lower net calcium absorption after a single dose of lanthanum carbonate compared with sevelamer carbonate. In conclusion, these two noncalcium-containing phosphate-binding agents showed a differential effect on gastrointestinal calcium absorption. These findings may help to improve the management of calcium balance in patients with renal failure, including concomitant use of vitamin D.

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

Objective

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

Data Sources

Human and veterinary literature.

Human Data Synthesis

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

Veterinary Data Synthesis

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

Conclusions

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

Disorders of calcium and magnesium balance: a physiology-based approach

Disorders of calcium and magnesium balance are physiologically interesting and clinically challenging. In this review, we attempt to bridge the gap between physiology and practice by providing a physiology-based approach to understanding hypocalcemia, hypercalcemia and hypomagnesemia. Calcium and, to a lesser extent, magnesium balance is achieved through a complex interplay between the parathyroid gland, bone, the intestine and the kidney. Our understanding of the molecular physiology of calcium and magnesium balance has grown considerably following the discovery of the calcium-sensing receptor (CaSR) and the main intestinal and renal transporters for calcium and magnesium, namely, the transient receptor potential channels TRPV5, TRPV6 and TRPM6. The regulation of parathyroid hormone (PTH) secretion by CaSR and the subsequent effects of PTH and vitamin D on TRPV5 constitute an increasingly characterized regulatory loop. In contrast, no truly magnesiotropic hormones have been identified, although the recently established interactions between the epidermal growth factor and TRPM6 suggest a possible candidate. Overall, the aim of this review is to illustrate the clinical disorders of calcium and magnesium balance from the perspective of their integrated physiology.

Regulation of calcium influx and signaling pathway in cancer cells via TRPV6-Numb1 interaction

Ca(2+) is a critical factor in the regulation of signal transduction and Ca(2+) homeostasis is altered in different human diseases. The level of Ca(2+) in cells is highly regulated through a diverse class of regulators. Among them is the transient receptor potential vanilloid 6 (TRPV6), which is a Ca(2+) selective channel that absorbs Ca(2+) in the small intestine. TRPV6 is overexpressed in some cancers and exhibits oncogenic potential, but its exact mechanism is still poorly understood. The Numb protein is a cell fate determinant that functions in endocytosis and as a tumor suppressor via the stabilization of p53. Numb protein consisted of four isoforms. Here, we showed a novel function of Numb1, which negatively regulates TRPV6 activity. The expression of Numb1 decreased cytosolic Ca(2+) concentrations in TRPV6-transfected HEK293 cells. When all the isoforms of Numb were depleted using siRNA in a TRPV6 stable cell line, the levels of cytosolic Ca(2+) increased. We observed an interaction between Numb1 and TRPV6 using co-immunoprecipitation. We confirmed this interaction using Fluorescence Resolution Energy Transfer (FRET). We identified the TRPV6 and Numb1 binding site using TRPV6 C-terminal truncation mutants and Numb1 deletion mutants. The binding site in TRPV6 was an aspartic acid at amino acid residue 716, and that binding site in Numb1 was arginine at amino acid residue 434. A Numb1 mutant, lacking TRPV6 binding activity, failed to inhibit TRPV6 activity. Every isoform of Numb knockdown, using an siRNA-based approach in MCF-7 breast cancer cells, not only showed enhanced TRPV6 expression but also both the cytosolic Ca(2+) concentration and cell proliferation were increased. The down-regulated expression of TRPV6 using siRNA increased Numb protein expression; however, the cytosolic influx of Ca(2+) and proliferation of the cell were decreased. To examine downstream signaling during Ca(2+) influx, we performed Western blotting analysis on TRPV6 upregulated cancer cells (MCF-7, PC-3). Taken together, these results demonstrated that Numb1 interacts with TRPV6 through charged residues and inhibits its activity via the regulation of protein expression. Moreover, we provided evidence for a Ca(2+)-regulated cancer cell signaling pathway and that the Ca(2+) channel is a target of cancer cells.

Inhibition of the human epithelial calcium channel TRPV6 by 2-aminoethoxydiphenyl borate (2-APB)

TRPV6, a highly calcium-selective member of the transient receptor potential (TRP) channel superfamily, is a major pathway for calcium absorption in the fetal and adult body. It is expressed abundantly in the duodenum, the placenta and exocrine tissues. TRVP6 was postulated to contribute to store-operated calcium channel (SOC) activity in certain cell types such as exocrine cells. In this study, we tested 2-APB, a widely used SOC inhibitor on human TRPV6 (hTRPV6) activity using fluorescence imaging, patch clamp and radioactive tracer techniques in transiently and stably transfected HEK293 cells. We found that the basal calcium and cadmium influx was higher in HEK293 cells transfected with hTRPV6 than in non-transfected cells. 2-APB inhibited hTRPV6 activity in both transient and stably transfected cells. This effect was slightly sensitive toward extracellular calcium. The extracellular sodium concentration did not affect the inhibition of hTRPV6 by 2-APB. However, N-methyl-d-glucamine significantly diminished the inhibitory effect of 2-APB presumably through direct interaction with this compound. Furthermore, 2-APB inhibited the activity of TRPV6 orthologs but not human TRPV5. 2-APB may serve as a parental compound for the development of therapeutic strategies specifically targeting the hTRPV6 calcium channel.

Metabolic complications associated with use of diuretics

Diuretics are commonly used therapeutic agents that act to inhibit sodium transport systems along the length of the renal tubule. The most effective diuretics are inhibitors of sodium chloride transport in the thick ascending limb of Henle. Loop diuretics mobilize large amounts of sodium chloride and water and produce a copious diuresis with a sharp reduction of extracellular fluid volume. As the site of action of diuretics moves downstream (thiazide and potassium-sparing diuretics), their effectiveness declines because the transport systems they inhibit have low transport capacity. Depending on the site of action diuretics can influence the renal handling of electrolyte-free water, calcium, potassium, protons, sodium bicarbonate, and uric acid. As a result, electrolyte and acid-base disorders commonly accompany diuretic use. Glucose and lipid abnormalities also can occur, particularly with the use of thiazide diuretics. This review focuses on the biochemical complications associated with the use of diuretics. The development of these complications can be minimized with careful monitoring, dosage adjustment, and replacement of electrolyte losses.

Regulation of target gene expression by the vitamin D receptor - an update on mechanisms

Virtually all of the known biological actions of the hormonal ligand 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) are mediated by the vitamin D receptor (VDR). Following binding and activation by the ligand, the VDR localizes in the nucleus to the regulatory regions of target genes and recruits chromatin-active coregulatory complexes which, in turn, modulate transcriptional output. The failure of the VDR to function due to crippling mutations results in total hereditary resistance to 1,25(OH)(2)D(3) in both mice and humans. In this review, we summarize the structural and functional properties of the VDR and the role of 1,25(OH)(2)D(3) in receptor activation, and then describe the results of recent studies using genome-wide analyses that define the overarching principles through which the VDR modulates genes expression. We also focus on the recent analysis of a specific 1,25(OH)(2)D(3) regulated gene that provides confirmation of the principles identified through these genome-wide methodologies. Taken together, these studies suggest an unanticipated increase in the complexity of the molecular processes that govern gene regulation by hormones and other factors.