The transient receptor potential cation channel subfamily V member 6 (TRPV6): genetics, biochemical properties, and functions of exceptional calcium channel proteins

Functionally deficient TRPV6 variants contribute to hereditary and familial chronic pancreatitis

The recent discovery of TRPV6 as a pancreatitis susceptibility gene served to identify a novel mechanism of chronic pancreatitis (CP) due to Ca²⁺ dysregulation. Herein, we analyzed TRPV6 in 81 probands with hereditary CP (HCP), 204 probands with familial CP (FCP), and 462 patients with idiopathic CP (ICP) by targeted next-generation sequencing. We identified 25 rare nonsynonymous TRPV6 variants, 18 of which had not been previously reported. All 18 variants were characterized by a Ca²⁺ imaging assay, with 8 being identified as functionally deficient. Evaluation of functionally deficient variants in the three CP cohorts revealed two novel findings: (i) functionally deficient TRPV6 variants appear to occur more frequently in HCP/FCP patients than in ICP patients (3.2% vs. 1.5%) and (ii) functionally deficient TRPV6 variants found in HCP and FCP probands appear to be more frequently coinherited with known risk variants in SPINK1, CTRC, and/or CFTR than those found in ICP patients (66.7% vs 28.6%). Additionally, genetic analysis of available HCP and FCP family members revealed complex patterns of inheritance in some families. Our findings confirm that functionally deficient TRPV6 variants represent an important contributor to CP. Importantly, functionally deficient TRPV6 variants account for a significant proportion of cases of HCP/FCP.

Transcriptomic Analysis of Salt-Stress-Responsive Genes in Barley Roots and Leaves

Barley is characterized by a rich genetic diversity, making it an important model for studies of salinity response with great potential for crop improvement. Moreover, salt stress severely affects barley growth and development, leading to substantial yield loss. Leaf and root transcriptomes of a salt-tolerant Tunisian landrace (Boulifa) exposed to 2, 8, and 24 h salt stress were compared with pre-exposure plants to identify candidate genes and pathways underlying barley's response. Expression of 3585 genes was upregulated and 5586 downregulated in leaves, while expression of 13,200 genes was upregulated and 10,575 downregulated in roots. Regulation of gene expression was severely impacted in roots, highlighting the complexity of salt stress response mechanisms in this tissue. Functional analyses in both tissues indicated that response to salt stress is mainly achieved through sensing and signaling pathways, strong transcriptional reprograming, hormone osmolyte and ion homeostasis stabilization, increased reactive oxygen scavenging, and activation of transport and photosynthesis systems. A number of candidate genes involved in hormone and kinase signaling pathways, as well as several transcription factor families and transporters, were identified. This study provides valuable information on early salt-stress-responsive genes in roots and leaves of barley and identifies several important players in salt tolerance.

Transient Receptor Potential Vanilloid 6 (TRPV6) Proteins Control the Extracellular Matrix Structure of the Placental Labyrinth

Calcium-selective transient receptor potential Vanilloid 6 (TRPV6) channels are expressed in fetal labyrinth trophoblasts as part of the feto-maternal barrier, necessary for sufficient calcium supply, embryo growth, and bone development during pregnancy. Recently, we have shown a less- compact labyrinth morphology of Trpv6-deficient placentae, and reduced Ca²⁺ uptake of primary trophoblasts upon functional deletion of TRPV6. Trpv6^-/- trophoblasts show a distinct calcium-dependent phenotype. Deep proteomic profiling of wt and Trpv6^-/- primary trophoblasts using label-free quantitative mass spectrometry leads to the identification of 2778 proteins. Among those, a group of proteases, including high-temperature requirement A serine peptidase 1 (HTRA1) and different granzymes are more abundantly expressed in Trpv6^-/- trophoblast lysates, whereas the extracellular matrix protein fibronectin and the fibronectin-domain-containing protein 3A (FND3A) were markedly reduced. Trpv6^-/-placenta lysates contain a higher intrinsic proteolytic activity increasing fibronectin degradation. Our results show that the extracellular matrix formation of the placental labyrinth depends on TRPV6; its deletion in trophoblasts correlates with the increased expression of proteases controlling the extracellular matrix in the labyrinth during pregnancy.

Manganese-enhanced MRI (MEMRI) in breast and prostate cancers: Preliminary results exploring the potential role of calcium receptors

Procedures

To preliminary assess the relationship between Manganese Enhanced Magnetic Resonance Imaging (MEMRI) and the expression of calcium receptors in human prostate and breast cancer animal models.

Methods

NOD/SCID mice were inoculated with MDA-MB-231 breast cancer cells and prostate PC3 cancer cells to develop orthotopic or pseudometastatic cancer animal models. Mice were studied on a clinical 3T scanner by using a prototype birdcage coil before and after intravenous injection of MnCl₂. Assessment of receptor’s status was carried out after the MR images acquisition by immunohistochemistry on excised tumours.

Results

Manganese contrast enhancement in breast or prostate cancer animal models well correlated with CaSR expression (p<0.01), whereas TRPV6 expression levels appeared not relevant to the Mn uptake.

Conclusion

Our preliminary results suggest that MEMRI appears an efficient tool to characterize human breast and prostate cancer animal models in the presence of different expression level of calcium receptors.

Variants That Affect Function of Calcium Channel TRPV6 Are Associated With Early-Onset Chronic Pancreatitis

BACKGROUND & AIMS

Changes in pancreatic calcium levels affect secretion and might be involved in development of chronic pancreatitis (CP). We investigated the association of CP with the transient receptor potential cation channel subfamily V member 6 gene (TRPV6), which encodes a Ca²⁺-selective ion channel, in an international cohort of patients and in mice.

METHODS

We performed whole-exome DNA sequencing from a patient with idiopathic CP and from his parents, who did not have CP. We validated our findings by sequencing DNA from 300 patients with CP (not associated with alcohol consumption) and 1070 persons from the general population in Japan (control individuals). In replication studies, we sequenced DNA from patients with early-onset CP (20 years or younger) not associated with alcohol consumption from France (n = 470) and Germany (n = 410). We expressed TRPV6 variants in HEK293 cells and measured their activity using Ca²⁺ imaging assays. CP was induced by repeated injections of cerulein in TRPV6^mut/mut mice.

RESULTS

We identified the variants c.629C>T (p.A210V) and c.970G>A (p.D324N) in TRPV6 in the index patient. Variants that affected function of the TRPV6 product were found in 13 of 300 patients (4.3%) and 1 of 1070 control individuals (0.1%) from Japan (odds ratio [OR], 48.4; 95% confidence interval [CI], 6.3-371.7; P = 2.4 × 10^-8). Twelve of 124 patients (9.7%) with early-onset CP had such variants. In the replication set from Europe, 18 patients with CP (2.0%) carried variants that affected the function of the TRPV6 product compared with 0 control individuals (P = 6.2 × 10^-8). Variants that did not affect the function of the TRPV6 product (p.I223T and p.D324N) were overrepresented in Japanese patients vs control individuals (OR, 10.9; 95% CI, 4.5-25.9; P = 7.4 × 10^-9 for p.I223T and P = .01 for p.D324N), whereas the p.L299Q was overrepresented in European patients vs control individuals (OR, 3.0; 95% CI, 1.9-4.8; P = 1.2 × 10^-5). TRPV6^mut/mut mice given cerulein developed more severe pancreatitis than control mice, as shown by increased levels of pancreatic enzymes, histologic alterations, and pancreatic fibrosis.

CONCLUSIONS

We found that patients with early-onset CP not associated with alcohol consumption carry variants in TRPV6 that affect the function of its product, perhaps by altering Ca²⁺ balance in pancreatic cells. TRPV6 regulates Ca²⁺ homeostasis and pancreatic inflammation.

Sperm motility modulated by Trpv1 regulates zebrafish fertilization

By responding to environmental and intracellular stimuli, ion channels play critical roles in sperm function regulation. Although the importance of ion channel in male reproduction has drawn increasing attention in many species, the knowledge about ion channels in zebrafish sperm is limited. Here, we show zebrafish sperm motility could be suppressed by general calcium channel blockers rather than by general potassium channel blockers. Further investigation found that sperm motility was not only suppressed by antagonist for the transient receptor potential vanilloid channel, subtype 1 (Trpv1), but also restored by its agonist, suggesting functional presence of Trpv1 in zebrafish spermatozoa. As a consequence, the suppression of sperm motility by Trpv1 antagonist could reduce in vitro fertilization rate. Western blot and immunofluorescence analysis proved that Trpv1 was mainly distributed in the neck and tail regions of zebrafish sperm. Additionally, neither antagonist nor agonist of Trpv1 exhibited effect on the motility of trpv1^-/- zebrafish sperm. To our knowledge, this is one of the limited studies showing the importance of ion channels in regulating zebrafish sperm function, and may enrich our understanding on male reproductive physiology of zebrafish and offer novel regulatory target for fish breeding and sperm cryopreservation.

TRPV6 as A Target for Cancer Therapy

Two decades ago a class of ion channels, hitherto unsuspected, was discovered. In mammals these Transient Receptor Potential channels (TRPs) have not only expanded in number (to 26 functional channels) but also expanded the view of our interface with the physical and chemical environment. Some are heat and cold sensors while others monitor endogenous and/or exogenous chemical signals. Some TRP channels monitor osmotic potential, and others measure cell movement, stretching, and fluid flow. Many TRP channels are major players in nociception and integration of pain signals. One member of the vanilloid sub-family of channels is TRPV6. This channel is highly selective for divalent cations, particularly calcium, and plays a part in general whole-body calcium homeostasis, capturing calcium in the gut from the diet. TRPV6 can be greatly elevated in a number of cancers deriving from epithelia and considerable study has been made of its role in the cancer phenotype where calcium control is dysfunctional. This review compiles and updates recent published work on TRPV6 as a promising drug target in a number of cancers including those afflicting breast, ovarian, prostate and pancreatic tissues.

Coupling of TRPV6 and TMEM16A in epithelial principal cells of the rat epididymis

Principal cells regulate the ionic environment of the epididymal lumen via unknown mechanisms. Gao et al. use electrophysiological and pharmacological tools to characterize rat principal cells and reveal a TRPV6-mediated calcium conductance and TMEM16A-mediated calcium-activated chloride conductance.

The epididymis establishes a congenial environment for sperm maturation and protection. Its fluid is acidic, and the calcium concentration is low and declines along the length of the epididymal tubule. However, our knowledge of ionic currents and mechanisms of calcium homeostasis in rat epididymal epithelial cells remains enigmatic. In this study, to better understand calcium regulation in the epididymis, we use the patch-clamp method to record from single rat cauda epididymal principal cells. We detect a constitutively active Ca²⁺ current with characteristics that match the epithelial calcium channel TRPV6. Electrophysiological and pharmacological data also reveal a constitutively active calcium-activated chloride conductance (CaCC). Removal of extracellular calcium attenuates not only the TRPV6-like conductance, but also the CaCC. Lanthanide block is time dependent such that the TRPV6-like component is inhibited first, followed by the CaCC. The putative CaCC blocker niflumic acid partially inhibits whole-cell currents, whereas La³⁺ almost abolishes whole-cell currents in principal cells. Membrane potential measurements reveal an interplay between La³⁺-sensitive ion channels and those that are sensitive to the specific TMEM16A inhibitor tannic acid. In vivo perfusion of the cauda epididymal tubule shows a substantial rate of Ca²⁺ reabsorption from the luminal side, which is dose-dependently suppressed by ruthenium red, a putative blocker of epithelial Ca²⁺ channels and CaCC. Finally, we discover messenger RNA for both TRPV6 and TMEM16A in the rat epididymis and show that their proteins colocalize in the apical membrane of principal cells. Collectively, these data provide evidence for a coupling mechanism between TRPV6 and TMEM16A in principal cells that may play an important role in the regulation of calcium homeostasis in the epididymis.