ATP modulates Ca2+ uptake by TRPV6 and is counteracted by isoform-specific phosphorylation

Microalgae polyphosphate nanoparticles deliver bioavailable calcium against phytate antagonism: Ex vivo and in vivo studies

Biogenic nanoparticles are promising carriers to deliver essential minerals. Here, calcium-enriched polyphosphate nanoparticles (CaPNPs) with a Ca/P molar ratio > 0.5 were produced by Synechococcus sp. PCC 7002 in the growth medium containing 1.08 g/L CaCl2, and had nearly spherical morphologies with a wide size distribution of 5-75 nm and strongly anionic surface properties with an average ζ-potential of -39 mV, according to dynamic light-scattering analysis, transmission and scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The ex-vivo ligated mouse ileal loop assays found that calcium in CaPNPs was readily available to intestinal absorption via both ion channel-mediated and endocytic pathways, specifically invoking macropinocytic internalization, lysosomal degradation, and transcytosis. Rat oral pharmacokinetics revealed that CaPNPs had a calcium bioavailability approximately 100 % relative to that of CaCl2 and more than 1.6 times of that of CaCO3. CaPNPs corrected the retinoic acid-induced increase in serum calcium, phosphorus, and bone-specific alkaline phosphatase, and decrease in serum osteocalcin, bone mineral content/density, and femoral geometric parameters with an efficacy equivalent to CaCl2 and markedly greater than CaCO3. In contrast to CaCl2, CaPNPs possessed desirable resistance against phytate's antagonistic action on calcium absorption in these ex vivo and in vivo studies. Overall, CaPNPs are attractive as a candidate agent for calcium supplementation, especially to populations on high-phytate diets.

Modulation of TRPA1 channel activity by Cdk5 in sensory neurons

Transient receptor potential cation channel, subfamily A, member 1 (TRPA1), is activated by a broad range of noxious stimuli. Cdk5, a member of the Cdk family, has recently been identified as a modulator of pain signaling pathways. In the current study, we investigated the extent to which Cdk5 modulates TRPA1 activity. Cdk5 inhibition was found to attenuate TRPA1 response to agonist in mouse DRG sensory neurons. Additionally, the presence of active Cdk5 was associated with increased TRPA1 phosphorylation in transfected HEK293 cells that was roscovitine-sensitive and absent in the mouse mutant S449A full-length channel. Immunopurified Cdk5 was observed to phosphorylate human TRPA1 peptide substrate at S448A in vitro. Our results point to a role for Cdk5 in modulating TRPA1 activity.

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.

Recent advances in therapeutic strategies that focus on the regulation of ion channel expression

A number of different ion channel types are involved in cell signaling networks, and homeostatic regulatory mechanisms contribute to the control of ion channel expression. Profiling of global gene expression using microarray technology has recently provided novel insights into the molecular mechanisms underlying the homeostatic and pathological control of ion channel expression. It has demonstrated that the dysregulation of ion channel expression is associated with the pathogenesis of neural, cardiovascular, and immune diseases as well as cancers. In addition to the transcriptional, translational, and post-translational regulation of ion channels, potentially important evidence on the mechanisms controlling ion channel expression has recently been accumulated. The regulation of alternative pre-mRNA splicing is therefore a novel therapeutic strategy for the treatment of dominant-negative splicing disorders. Epigenetic modification plays a key role in various pathological conditions through the regulation of pluripotency genes. Inhibitors of pre-mRNA splicing and histone deacetyalase/methyltransferase have potential as potent therapeutic drugs for cancers and autoimmune and inflammatory diseases. Moreover, membrane-anchoring proteins, lysosomal and proteasomal degradation-related molecules, auxiliary subunits, and pharmacological agents alter the protein folding, membrane trafficking, and post-translational modifications of ion channels, and are linked to expression-defect channelopathies. In this review, we focused on recent insights into the transcriptional, spliceosomal, epigenetic, and proteasomal regulation of ion channel expression: Ca(2+) channels (TRPC/TRPV/TRPM/TRPA/Orai), K(+) channels (voltage-gated, KV/Ca(2+)-activated, KCa/two-pore domain, K2P/inward-rectifier, Kir), and Ca(2+)-activated Cl(-) channels (TMEM16A/TMEM16B). Furthermore, this review highlights expression of these ion channels in expression-defect channelopathies.

A STIM2 splice variant negatively regulates store-operated calcium entry

Cellular homeostasis relies upon precise regulation of Ca(2+) concentration. Stromal interaction molecule (STIM) proteins regulate store-operated calcium entry (SOCE) by sensing Ca(2+) concentration in the ER and forming oligomers to trigger Ca(2+) entry through plasma membrane-localized Orai1 channels. Here we characterize a STIM2 splice variant, STIM2.1, which retains an additional exon within the region encoding the channel-activating domain. Expression of STIM2.1 is ubiquitous but its abundance relative to the more common STIM2.2 variant is dependent upon cell type and highest in naive T cells. STIM2.1 knockdown increases SOCE in naive CD4(+) T cells, whereas knockdown of STIM2.2 decreases SOCE. Conversely, overexpression of STIM2.1, but not STIM2.2, decreases SOCE, indicating its inhibitory role. STIM2.1 interaction with Orai1 is impaired and prevents Orai1 activation, but STIM2.1 shows increased affinity towards calmodulin. Our results imply STIM2.1 as an additional player tuning Orai1 activation in vivo.

Adenosine triphosphate regulates the activity of guinea pig Cav1.2 channel by direct binding to the channel in a dose-dependent manner

The present study is to investigate the mechanism by which ATP regulates Cav1.2 channel activity. Ventricular tissue was obtained from adult guinea pig hearts using collagenase. Ca(2+) channel activity was monitored using the patch-clamp technique. Proteins were purified using wheat germ agglutinin-Sepharose, and the concentration was determined using the Coomassie brilliant blue technique. ATP binding to the Cav1.2 channel was examined using the photoaffinity method. EDA-ATP-biotin maintains Ca(2+) channel activity in inside-out membrane patches. ATP directly bound to the Cav1.2 channel in a dose-dependent manner, and at least two molecules of ATP bound to one molecule of the Cav1.2 channel. Low levels of calmodulin (CaM) increased ATP binding to the Cav1.2 channel, but higher levels of CaM decreased ATP binding to the Cav1.2 channel. In addition, Ca(2+) was another regulator for ATP binding to the Cav1.2 channel. Furthermore, ATP bound to GST-fusion peptides of NH2-terminal region (amino acids 6-140) and proximal COOH-terminal region (amino acids 1,509-1,789) of the main subunit (α1C) of the Cav1.2 channel. Our data suggest that ATP might regulate Cav1.2 channel activity by directly binding to the Cav1.2 channel in a dose-dependent manner. In addition, the ATP-binding effect to the Cav1.2 channel was both CaM- and Ca(2+) dependent.

Structure of thermally activated TRP channels

Temperature sensation is important for adaptation and survival of organisms. While temperature has the potential to affect all biological macromolecules, organisms have evolved specific thermosensitive molecular detectors that are able to transduce temperature changes into physiologically relevant signals. Among these thermosensors are ion channels from the transient receptor potential (TRP) family. Prime candidates include TRPV1-4, TRPA1, and TRPM8 (the so-called "thermoTRP" channels), which are expressed in sensory neurons and gated at specific temperatures. Electrophysiological and thermodynamic approaches have been employed to determine the nature by which thermoTRPs detect temperature and couple temperature changes to channel gating. To further understand how thermoTRPs sense temperature, high-resolution structures of full-length thermoTRPs channels will be required. Here, we will discuss current progress in unraveling the structures of thermoTRP channels.

TRPV6 channels

TRPV6 (former synonyms ECAC2, CaT1, CaT-like) displays several specific features which makes it unique among the members of the mammalian Trp gene family (1) TRPV6 (and its closest relative, TRPV5) are the only highly Ca(2+)-selective channels of the entire TRP superfamily (Peng et al. 1999; Wissenbach et al. 2001; Voets et al. 2004). (2) Translation of Trpv6 initiates at a non-AUG codon, at ACG, located upstream of the annotated AUG, which is not used for initiation (Fecher-Trost et al. 2013). The ACG codon is nevertheless decoded by methionine. Not only a very rare event in eukaryotic biology, the full-length TRPV6 protein existing in vivo comprises an amino terminus extended by 40 amino acid residues compared to the annotated truncated TRPV6 protein which has been used in most studies on TRPV6 channel activity so far. (In the following numbering occurs according to this full-length protein, with the numbers of the so far annotated truncated protein in brackets). (3) Only in humans a coupled polymorphism of Trpv6 exists causing three amino acid exchanges and resulting in an ancestral Trpv6 haplotype and a so-called derived Trpv6 haplotype (Wissenbach et al. 2001). The ancestral allele encodes the amino acid residues C197(157), M418(378) and M721(681) and the derived alleles R197(157), V418(378) and T721(681). The ancestral haplotype is found in all species, the derived Trpv6 haplotype has only been identified in humans, and its frequency increases with the distance to the African continent. Apparently the Trpv6 gene has been a strong target for selection in humans, and its derived variant is one of the few examples showing consistently differences to the orthologues genes of other primates (Akey et al. 2004, 2006; Stajich and Hahn 2005; Hughes et al. 2008). (4) The Trpv6 gene expression is significantly upregulated in several human malignancies including the most common cancers, prostate and breast cancer (Wissenbach et al. 2001; Zhuang et al. 2002; Fixemer et al. 2003; Bolanz et al. 2008). (5) Male mice lacking functional TRPV6 channels are hypo-/infertile making TRPV6 one of the very few channels essential for male fertility (Weissgerber et al. 2011, 2012).

Structural insight into the assembly of TRPV channels

Transient receptor potential (TRP) proteins are a large family of polymodal nonselective cation channels. The TRP vanilloid (TRPV) subfamily consists of six homologous members with diverse functions. TRPV1-TRPV4 are nonselective cation channels proposed to play a role in nociception, while TRPV5 and TRPV6 are involved in epithelial Ca²⁺ homeostasis. Here we present the cryo-electron microscopy (cryo-EM) structure of functional, full-length TRPV2 at 13.6 Å resolution. The map reveals that the TRPV2 cytoplasmic domain displays a 4-fold petal-like shape in which high-resolution N-terminal ankyrin repeat domain (ARD) structures can be unambiguously fitted. Fitting of the available ARD structures for other TRPV subfamily members into the TRPV2 EM map suggests that TRPV subfamily members have highly homologous structural topologies. These results allowed us to postulate a structural explanation for the functional diversity among TRPV channels and their differential regulation by proteins and ligands.

Lipid signaling cascades of orexin/hypocretin receptors

Orexins - orexin-A and orexin-B - are neuropeptides with significant role in regulation of fundamental physiological processes such as sleep-wakefulness cycle. Orexins act via G-protein-coupled OX1 and OX2 receptors, which are found, in addition to the central nervous system, also in a number of peripheral organs. Orexin receptors show high degree of signaling promiscuity. One particularly prominent way of signaling for these receptors is via phospholipase cascades, including the phospholipase C, phospholipase D and phospholipase A2 cascades, and also diacylglycerol lipase and phosphoinositide-3-kinase pathways. Most analyses have been performed in recombinant cells; there are indications of some of these cascades in native cells while the significance of other cascades remains to be shown. In this review, I present these pathways, their activation mechanisms and their physiological significance.

In vivo detection of human TRPV6-rich tumors with anti-cancer peptides derived from soricidin

Soricidin is a 54-amino acid peptide found in the paralytic venom of the northern short-tailed shrew (Blarina brevicauda) and has been found to inhibit the transient receptor potential of vallinoid type 6 (TRPV6) calcium channels. We report that two shorter peptides, SOR-C13 and SOR-C27, derived from the C-terminus of soricidin, are high-affinity antagonists of human TRPV6 channels that are up-regulated in a number of cancers. Herein, we report molecular imaging methods that demonstrate the in vivo diagnostic potential of SOR-C13 and SOR-C27 to target tumor sites in mice bearing ovarian or prostate tumors. Our results suggest that these novel peptides may provide an avenue to deliver diagnostic and therapeutic reagents directly to TRPV6-rich tumors and, as such, have potential applications for a range of carcinomas including ovarian, breast, thyroid, prostate and colon, as well as certain leukemia's and lymphomas.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Decrease in phosphatidylinositol 4,5-bisphosphate levels mediates desensitization of the cold sensor TRPM8 channels

The activity of the cold- and menthol-activated transient receptor potential melastatin 8 (TRPM8) channels diminishes over time in the presence of extracellular Ca(2+), a phenomenon referred to as desensitization or adaptation. Here we show that activation of TRPM8 by cold or menthol evokes a decrease in cellular phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] levels. The decrease in PtdIns(4,5)P(2) levels was accompanied by increased inositol 1,4,5 trisphosphate (InsP(3)) production, and was inhibited by loading the cells with the Ca(2+) chelator BAPTA-AM, showing that it was the consequence of the activation of phospholipase C (PLC) by increased intracellular Ca(2+) concentrations. PtdIns(4,5)P(2) hydrolysis showed excellent temporal correlation with current desensitization in simultaneous patch clamp and fluorescence-based PtdIns(4,5)P(2) level measurements. Intracellular dialysis of PtdIns(4,5)P(2) inhibited desensitization both in native neuronal and recombinant TRPM8 channels. PtdIns(4)P, the precursor of PtdIns(4,5)P(2), did not inhibit desensitization, consistent with its minimal effect in excised patches. Omission of MgATP from the intracellular solution accelerated desensitization, and MgATP reactivated TRPM8 channels in excised patches in a phosphatidylinositol 4-kinase (PI4K)-dependent manner. PLC-independent depletion of PtdIns(4,5)P(2) using a voltage-sensitive phosphatase (ci-VSP) inhibited TRPM8 currents, and omission of ATP from the intracellular solution inhibited recovery from this inhibition. Inhibitors of PKC had no effect on the kinetics of desensitization. We conclude that Ca(2+) influx through TRPM8 activates a Ca(2+)-sensitive PLC isoform, and the resulting depletion of PtdIns(4,5)P(2) plays a major role in desensitization of both cold and menthol responses.

TRPV channels and vascular function

Transient receptor potential channels, of the vanilloid subtype (TRPV), act as sensory mediators, being activated by endogenous ligands, heat, mechanical and osmotic stress. Within the vasculature, TRPV channels are expressed in smooth muscle cells, endothelial cells, as well as in peri-vascular nerves. Their varied distribution and polymodal activation properties make them ideally suited to a role in modulating vascular function, perceiving and responding to local environmental changes. In endothelial cells, TRPV1 is activated by endocannabinoids, TRPV3 by dietary agonists and TRPV4 by shear stress, epoxyeicosatrienoic acids (EETs) and downstream of Gq-coupled receptor activation. Upon activation, these channels contribute to vasodilation via nitric oxide, prostacyclin and intermediate/small conductance potassium channel-dependent pathways. In smooth muscle, TRPV4 is activated by endothelial-derived EETs, leading to large conductance potassium channel activation and smooth muscle hyperpolarization. Conversely, smooth muscle TRPV2 channels contribute to global calcium entry and may aid constriction. TRPV1 and TRPV4 are expressed in sensory nerves and can cause vasodilation through calcitonin gene-related peptide and substance P release as well as mediating vascular function via the baroreceptor reflex (TRPV1) or via increasing sympathetic outflow during osmotic stress (TRPV4). Thus, TRPV channels play important roles in the regulation of normal and pathological cellular function in the vasculature.

Intracellular ATP supports TRPV6 activity via lipid kinases and the generation of PtdIns(4,5) P₂

Transient receptor potential vanilloid 6 (TRPV6) channels play an important role in Ca(2+) absorption in the intestines. Both phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] and cytoplasmic ATP have been proposed to be important for maintaining TRPV6 activity. To evaluate whether PtdIns(4,5)P(2) and ATP affect channel activity directly or indirectly, we have used a dual approach, examining channel activity in excised patches and planar lipid bilayers. In excised inside-out patch-clamp measurements, ATP reactivated the human TRPV6 channels after current rundown only in the presence of Mg(2+). The effect of MgATP was inhibited by 3 structurally different compounds that inhibit type III phosphatidylinositol 4-kinases (PI4Ks). PtdIns(4,5)P(2) also activated TRPV6 in excised patches, while its precursor PtdIns(4)P had only minimal effect. These data demonstrate that MgATP provides substrate for lipid kinases, allowing the resynthesis of PtdIns(4,5)P(2). To determine whether PtdIns(4,5)P(2) is a direct activator of TRPV6, we purified and reconstituted the channel protein in planar lipid bilayers. The reconstituted channel showed high activity in the presence of PtdIns(4,5)P(2), while PtdIns(4)P induced only minimal activity. Our data establish PtdIns(4,5)P(2) as a direct activator of TRPV6 and demonstrate that intracellular ATP regulates the channel indirectly as a substrate for type III PI4Ks.

Effects of 1α,25 dihydroxyvitamin D3 and testosterone on miRNA and mRNA expression in LNCaP cells

Background

There is evidence from epidemiological and in vitro studies that the biological effects of testosterone (T) on cell cycle and survival are modulated by 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) in prostate cancer. To investigate the cross talk between androgen- and vitamin D-mediated intracellular signaling pathways, the individual and combined effects of T and 1,25(OH)₂D₃ on global gene expression in LNCaP prostate cancer cells were assessed.

Results

Stringent statistical analysis identifies a cohort of genes that lack one or both androgen response elements (AREs) or vitamin D response elements (VDREs) in their promoters, which are nevertheless differentially regulated by both steroids (either additively or synergistically). This suggests that mechanisms in addition to VDR- and AR-mediated transcription are responsible for the modulation of gene expression. Microarray analysis shows that fifteen miRNAs are also differentially regulated by 1,25(OH)₂D₃ and T. Among these miR-22, miR-29ab, miR-134, miR-1207-5p and miR-371-5p are up regulated, while miR-17 and miR-20a, members of the miR-17/92 cluster are down regulated. A number of genes implicated in cell cycle progression, lipid synthesis and accumulation and calcium homeostasis are among the mRNA targets of these miRNAs. Thus, in addition to their well characterized effects on transcription, mediated by either or both cognate nuclear receptors, 1,25(OH)₂D₃ and T regulate the steady state mRNA levels by modulating miRNA-mediated mRNA degradation, generating attenuation feedback loops that result in global changes in mRNA and protein levels. Changes in genes involved in calcium homeostasis may have specific clinical importance since the second messenger Ca²⁺ is known to modulate various cellular processes, including cell proliferation, cell death and cell motility, which affects prostate cancer tumor progression and responsiveness to therapy.

Conclusions

These data indicate that these two hormones combine to drive a differentiated phenotype, and reinforce the idea that the age dependent decline in both hormones results in the de-differentiation of prostate tumor cells, which results in increased proliferation, motility and invasion common to aggressive tumors. These studies also reinforce the potential importance of miRNAs in prostate cancer progression and therapeutic outcomes.

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.

International Union of Basic and Clinical Pharmacology. LXXVI. Current progress in the mammalian TRP ion channel family

Transient receptor potential (TRP) channels are a large family of ion channel proteins, surpassed in number in mammals only by voltage-gated potassium channels. TRP channels are activated and regulated through strikingly diverse mechanisms, making them suitable candidates for cellular sensors. They respond to environmental stimuli such as temperature, pH, osmolarity, pheromones, taste, and plant compounds, and intracellular stimuli such as Ca(2+) and phosphatidylinositol signal transduction pathways. However, it is still largely unknown how TRP channels are activated in vivo. Despite the uncertainties, emerging evidence using TRP channel knockout mice indicates that these channels have broad function in physiology. Here we review the recent progress on the physiology, pharmacology and pathophysiological function of mammalian TRP channels.