TRPV6 compound heterozygous variants result in impaired placental calcium transport and severe undermineralization and dysplasia of the fetal skeleton

Structure-function analyses of human TRPV6 ancestral and derived haplotypes

TRPV6 is a Ca2+ selective channel that mediates calcium uptake in the gut and contributes to the development and progression of human cancers. TRPV6 is represented by the ancestral and derived haplotypes that differ by three non-synonymous polymorphisms, located in the N-terminal ankyrin repeat domain (C157R), S1-S2 extracellular loop (M378V), and C-terminus (M681T). The ancestral and derived haplotypes were proposed to serve as genomic factors causing a different outcome for cancer patients of African ancestry. We solved cryoelectron microscopy (cryo-EM) structures of ancestral and derived TRPV6 in the open and calmodulin (CaM)-bound inactivated states. Neither state shows substantial structural differences caused by the non-synonymous polymorphisms. Functional properties assessed by electrophysiological recordings and Ca2+ uptake measurements, and water and ion permeation evaluated by molecular modeling also appear similar between the haplotypes. Therefore, ancestral and derived TRPV6 have similar structure and function, implying that other factors are responsible for the differences in susceptibility to cancer.

Targeting TRP channels: recent advances in structure, ligand binding, and molecular mechanisms

Transient receptor potential (TRP) channels are a large and diverse family of transmembrane ion channels that are widely expressed, have important physiological roles, and are associated with many human diseases. These proteins are actively pursued as promising drug targets, benefitting greatly from advances in structural and mechanistic studies of TRP channels. At the same time, the complex, polymodal activation and regulation of TRP channels have presented formidable challenges. In this short review, we summarize recent progresses toward understanding the structural basis of TRP channel function, as well as potential ligand binding sites that could be targeted for therapeutics. A particular focus is on the current understanding of the molecular mechanisms of TRP channel activation and regulation, where many fundamental questions remain unanswered. We believe that a deeper understanding of the functional mechanisms of TRP channels will be critical and likely transformative toward developing successful therapeutic strategies targeting these exciting proteins. This endeavor will require concerted efforts from computation, structural biology, medicinal chemistry, electrophysiology, pharmacology, drug safety and clinical studies.

Molecular pharmacology of the onco-TRP channel TRPV6

TRPV6, a representative of the vanilloid subfamily of TRP channels, serves as the principal calcium uptake channel in the gut. Dysregulation of TRPV6 results in disturbed calcium homeostasis leading to a variety of human diseases, including many forms of cancer. Inhibitors of this oncochannel are therefore particularly needed. In this review, we provide an overview of recent advances in structural pharmacology that uncovered the molecular mechanisms of TRPV6 inhibition by a variety of small molecules, including synthetic and natural, plant-derived compounds as well as some prospective and clinically approved drugs.

Molecular pathway and structural mechanism of human oncochannel TRPV6 inhibition by the phytocannabinoid tetrahydrocannabivarin

The calcium-selective oncochannel TRPV6 is an important driver of cell proliferation in human cancers. Despite increasing interest of pharmacological research in developing synthetic inhibitors of TRPV6, natural compounds acting at this channel have been largely neglected. On the other hand, pharmacokinetics of natural small-molecule antagonists optimized by nature throughout evolution endows these compounds with a medicinal potential to serve as potent and safe next-generation anti-cancer drugs. Here we report the structure of human TRPV6 in complex with tetrahydrocannabivarin (THCV), a natural cannabinoid inhibitor extracted from Cannabis sativa. We use cryo-electron microscopy combined with electrophysiology, calcium imaging, mutagenesis, and molecular dynamics simulations to identify THCV binding sites in the portals that connect the membrane environment surrounding the protein to the central cavity of the channel pore and to characterize the allosteric mechanism of TRPV6 inhibition. We also propose the molecular pathway taken by THCV to reach its binding site. Our study provides a foundation for the development of new TRPV6-targeting drugs.

Structural mechanism of human oncochannel TRPV6 inhibition by the natural phytoestrogen genistein

Calcium-selective oncochannel TRPV6 is the major driver of cell proliferation in human cancers. While significant effort has been invested in the development of synthetic TRPV6 inhibitors, natural channel blockers have been largely neglected. Here we report the structure of human TRPV6 in complex with the plant-derived phytoestrogen genistein, extracted from Styphnolobium japonicum, that was shown to inhibit cell invasion and metastasis in cancer clinical trials. Despite the pharmacological value, the molecular mechanism of TRPV6 inhibition by genistein has remained enigmatic. We use cryo-EM combined with electrophysiology, calcium imaging, mutagenesis, and molecular dynamics simulations to show that genistein binds in the intracellular half of the TRPV6 pore and acts as an ion channel blocker and gating modifier. Genistein binding to the open channel causes pore closure and a two-fold symmetrical conformational rearrangement in the S4-S5 and S6-TRP helix regions. The unprecedented mechanism of TRPV6 inhibition by genistein uncovers new possibilities in structure-based drug design.

Biochemistry and pathophysiology of the Transient Potential Receptor Vanilloid 6 (TRPV6) calcium channel

TRPV6 is a Transient Receptor Potential Vanilloid (TRPV) cation channel with high selectivity for Ca²⁺ ions. First identified in 1999 in a search for the gene which mediates intestinal Ca²⁺ absorption, its far more extensive repertoire as a guardian of intracellular Ca²⁺ has since become apparent. Studies on TRPV6-deficient mice demonstrated additional important roles in placental Ca²⁺ transport, fetal bone development and male fertility. The first reports of inherited deficiency in newborn babies appeared in 2018, revealing its physiological importance in humans. There is currently strong evidence that TRPV6 also contributes to the pathogenesis of some common cancers. The recently reported association of TRPV6 deficiency with non-alcoholic chronic pancreatitis suggests a role in normal pancreatic function. Over time and with greater awareness of TRPV6, other disease-associations are likely to emerge. Powerful analytical tools have provided invaluable insights into the structure and operation of TRPV6. Its roles in Ca²⁺ signaling and carcinogenesis, and the use of channel inhibitors in cancer treatment are being intensively investigated. This review first briefly describes the biochemistry and physiology of the channel, and analytical methods used to investigate these. The focus subsequently shifts to the clinical disorders associated with abnormal expression and the underlying pathophysiology. The aims of this review are to increase awareness of this channel, and to draw together findings from a wide range of sources which may help to formulate new ideas for further studies.

What structures did, and did not, reveal about the function of the epithelial Ca2+ channels TRPV5 and TRPV6

Transient Receptor Potential Vanilloid 5 and 6 (TRPV5 and TRPV6) are Ca2+ selective epithelial ion channels. They are the products of a relatively recent gene duplication in mammals, and have high sequence homology to each other. Their functional properties are also much more similar to each other than to other members of the TRPV subfamily. They are both constitutively active, and this activity depends on the endogenous cofactor phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]. Both channels undergo Ca2+-induced inactivation, which is mediated by direct binding of the ubiquitous Ca2+ binding protein calmodulin (CaM) to the channels, and by a decrease in PI(4,5)P2 levels by Ca2+ -induced activation of phospholipase C (PLC). Recent cryo electron microscopy (cryo-EM) and X-ray crystallography structures provided detailed structural information for both TRPV5 and TRPV6. This review will discuss this structural information in the context of the function of these channels focusing on the mechanism of CaM inhibition, activation by PI(4,5)P2 and binding of pharmacological modulators.

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.

The newest TRP channelopathy: Gain of function TRPM3 mutations cause epilepsy and intellectual disability

Transient Receptor Potential Melastatin 3 (TRPM3) is a Ca2+ permeable nonselective cation channel, activated by heat and chemical agonists, such as the endogenous neuro-steroid Pregnenolone Sulfate (PregS) and the chemical compound CIM0216. TRPM3 is expressed in peripheral sensory neurons of the dorsal root ganglia (DRG), and its role in noxious heat sensation in mice is well established. TRPM3 is also expressed in a number of other tissues, including the brain, but its role there has been largely unexplored. Recent reports showed that two mutations in TRPM3 are associated with a developmental and epileptic encephalopathy, pointing to an important role of TRPM3 in the human brain. Subsequent reports found that the two disease-associated mutations increased basal channel activity, and sensitivity of the channel to activation by heat and chemical agonists. This review will discuss these mutations in the context of human diseases caused by mutations in other TRP channels, and in the context of the biophysical properties and physiological functions of TRPM3.

Mutations That Affect the Surface Expression of TRPV6 Are Associated with the Upregulation of Serine Proteases in the Placenta of an Infant

Recently, we reported a case of an infant with neonatal severe under-mineralizing skeletal dysplasia caused by mutations within both alleles of the TRPV6 gene. One mutation results in an in frame stop codon (R510stop) that leads to a truncated, nonfunctional TRPV6 channel, and the second in a point mutation (G660R) that, surprisingly, does not affect the Ca2+ permeability of TRPV6. We mimicked the subunit composition of the unaffected heterozygous parent and child by coexpressing the TRPV6 G660R and R510stop mutants and combinations with wild type TRPV6. We show that both the G660R and R510stop mutant subunits are expressed and result in decreased calcium uptake, which is the result of the reduced abundancy of functional TRPV6 channels within the plasma membrane. We compared the proteomic profiles of a healthy placenta with that of the diseased infant and detected, exclusively in the latter two proteases, HTRA1 and cathepsin G. Our results implicate that the combination of the two mutant TRPV6 subunits, which are expressed in the placenta of the diseased child, is responsible for the decreased calcium uptake, which could explain the skeletal dysplasia. In addition, placental calcium deficiency also appears to be associated with an increase in the expression of proteases.

Structural mechanisms of TRPV6 inhibition by ruthenium red and econazole

TRPV6 is a calcium-selective ion channel implicated in epithelial Ca2+ uptake. TRPV6 inhibitors are needed for the treatment of a broad range of diseases associated with disturbed calcium homeostasis, including cancers. Here we combine cryo-EM, calcium imaging, and mutagenesis to explore molecular bases of human TRPV6 inhibition by the antifungal drug econazole and the universal ion channel blocker ruthenium red (RR). Econazole binds to an allosteric site at the channel's periphery, where it replaces a lipid. In contrast, RR inhibits TRPV6 by binding in the middle of the ion channel's selectivity filter and plugging its pore like a bottle cork. Despite different binding site locations, both inhibitors induce similar conformational changes in the channel resulting in closure of the gate formed by S6 helices bundle crossing. The uncovered molecular mechanisms of TRPV6 inhibition can guide the design of a new generation of clinically useful inhibitors.

A TRPV6 expression atlas for the mouse

The transient receptor potential vanilloid 6 (TRPV6) channel is highly Ca²⁺-selective and has been implicated in mediating transcellular Ca²⁺ transport and thus maintaining the Ca²⁺ balance in the body. To characterize its physiological function(s), a detailed expression profile of the TRPV6 channel throughout the body is essential. Capitalizing on a recently established murine Trpv6-reporter strain, we identified primary TRPV6 channel-expressing cells in an organism-wide manner. In a complementary experimental approach, we characterized TRPV6 expression in different tissues of wild-type mice by TRPV6 immunoprecipitation (IP) followed by mass spectrometry analysis and correlated these data with the reporter gene expression. Taken together, we present a TRPV6 expression atlas throughout the entire body of juvenile and adult mice, providing a novel resource to investigate the role of TRPV6 channels in vivo.

Comments on the evolution of TRPV6

It is well known that not all biological findings derived from animals can be directly applied to humans. The TRPV6 protein may serve as an example which highlights these inter-species differences as an example of parallel evolutionary pathways. TRPV6 (and TRPV5) belong to a family of ion channels from the transient receptor potential group but are selectively permeable for Ca²⁺, in contrast to other members of the family. Sequences with recognizable similarity to TRPV6 can already be found in archaebacteria. These ancient sequences show clear similarity to the ion-conducting pore of TRPV6. Over the course of evolution, the duplication of the TRPV6 gene gave rise to TRPV5. Duplications of the complete genome as well as subsequent loss of genetic material have led to a variety of different TRPV5/6 combinations. In addition, there is an N-terminal extension of the protein in placental animals. This extension causes translation of TRPV6 to be initiated from an ACG codon. Inactivation of one TRPV6 allele can be correlated with alcohol-independent pancreatitis in humans while inactivation of both alleles leads to skeletal dysplasia of newborn babies. The latter effect is not observed in mice, implying that the effects due to perturbations in TRPV6 levels are much more pronounced in humans.

Human TRPV6-pathies caused by gene mutations

The TRP-family of ion channels consists of 27 members in humans. Most TRP channels are non- selective cation channels with the exception of TRPV5 and TRPV6 which exhibit a high permeability for Ca²⁺ ions. A functional channel is formed by 4 identical subunits [1]. A growing number of mutations are present in human TRPV6 genes which alter channel function and can lead to elevated blood levels of the parathyroid hormone accompanied by transient hyperparathyroidism. Recent publications suggest that TRPV6 mutations could also trigger non-alcoholic chronic pancreatitis. This review summarises the consequences of these mutations within the TRPV6 gene.

Endocrine Diseases of Newborn: Epidemiology, Pathogenesis, Therapeutic Options, and Outcome "Current Insights Into Disorders of Calcium and Phosphate in the Newborn"

The physiology and regulation of bone minerals in the fetus and the newborn is significantly different from children and adults. The bone minerals calcium, phosphate and magnesium are all maintained at higher concentrations in utero to achieve adequate bone accretion. This is an integral component of normal fetal development which facilitates safe neonatal transition to post-natal life. When deciphering the cause of bone mineral disorders in newborns, the potential differential diagnosis list is broad and complex, including several extremely rare conditions. Also, significant discoveries including new embryological molecular genetic transcription factors, the role of active placental mineral transport, and hormone regulation factors have changed the understanding of calcium and phosphate homeostasis in the fetus and the newborn. This article will guide clinicians through an updated review of calcium and phosphate physiology, then review specific conditions pertinent to successful neonatal care. Furthermore, with the advancement of increasingly rapid molecular genetic testing, genomics will continue to play a greater role in this area of fetal diagnostics and prognostication.

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.

Sex-specific maternal calcium requirements for the prevention of nonalcoholic fatty liver disease by altering the intestinal microbiota and lipid metabolism in the high-fat-diet-fed offspring mice

The significance of maternal appropriate calcium intakes for energy metabolism in the offspring has been recognized. Nonalcoholic fatty liver disease (NAFLD) is considered as the hepatic manifestation of metabolic syndrome. So in this study, we proposed that there were long-term effects of maternal calcium status on the progress of NAFLD by altering the intestinal microbiota and lipid metabolism with attention to potential sex differences among the mouse offspring. Thirty-four-week female C57BL/6 J mice were subjected to obtain low, normal and high calcium reproductive diets throughout the gestation and lactation. After weaning, both the male and female mouse offspring were fed with the high-fat diet for 16 weeks, with the normal diet as control. Biochemical indicators in the plasma and hepatic tissue were measured using ELISA or enzymatic methods. The expression of lipid metabolism, inflammatory and fibrosis related genes was determined by RT-PCR. The intestinal microbiota was analyzed by 16S rRNA high-throughput sequencing. Maternal normal and low calcium intake could, respectively, inhibit the progress of high-fat diet induced NAFLD in the male and female mouse offspring, which was characterized by the least lipid droplets, inflammatory infiltration and fibrosis, the lowest concentrations of free fatty acids and triglyceridethe lowest expression of genes involving in de novo lipogenesis and the highest expression of genes related to lipid oxidation and  hydrolysis, inflammatory, and fibrosis. Pyrosequencing of 16S rRNA genes revealed that the male mouse offspring with maternal normal calcium intake and the female mouse offspring with maternal low calcium intake, after the high-fat diet feeding, had distinct intestinal microbiota, which was closer to thosein mice with the normal diet feeding. Analysis of the functional features for the different microbiota was compatible with the expression of genes associated with lipogenesis, lipid oxidation and hydrolysis. Thus, there is a sex-specific manner for maternal calcium requirement to inhibit the progress of offspring NAFLD, that might be less for the female offspring and more for the male offspring.

Inactivation-mimicking block of the epithelial calcium channel TRPV6

Epithelial calcium channel TRPV6 plays vital roles in calcium homeostasis, and its dysregulation is implicated in multifactorial diseases, including cancers. Here, we study the molecular mechanism of selective nanomolar-affinity TRPV6 inhibition by (4-phenylcyclohexyl)piperazine derivatives (PCHPDs). We use x-ray crystallography and cryo-electron microscopy to solve the inhibitor-bound structures of TRPV6 and identify two types of inhibitor binding sites in the transmembrane region: (i) modulatory sites between the S1-S4 and pore domains normally occupied by lipids and (ii) the main site in the ion channel pore. Our structural data combined with mutagenesis, functional and computational approaches suggest that PCHPDs plug the open pore of TRPV6 and convert the channel into a nonconducting state, mimicking the action of calmodulin, which causes inactivation of TRPV6 channels under physiological conditions. This mechanism of inhibition explains the high selectivity and potency of PCHPDs and opens up unexplored avenues for the design of future-generation biomimetic drugs.

Novel TRPV6 mutations in the spectrum of transient neonatal hyperparathyroidism

Maternal-fetal calcium (Ca2+) transport in the placenta plays a critical role in maintaining fetal bone mineralization. Mutations in the gene encoding the transient receptor potential cation channel, subfamily V, member 6 (TRPV6) have been identified as causative mutations of transient neonatal hyperparathyroidism due to insufficient maternal-fetal Ca2+ transport in the placenta. In this study, we found two novel mutations in subjects that have transient neonatal hyperparathyroidism. TRPV6 carrying the mutation p.Arg390His that localizes to the outer edge of the first transmembrane domain (S1) showed impaired trafficking to the plasma membrane, whereas TRPV6 having the mutation p.Gly291Ser in the sixth ankyrin repeat (AR) domain had channel properties that were comparable those of WT channels, although the increases in steady-state intracellular Ca2+ concentration could have led to Ca2+ overload and subsequent death of cells expressing this mutant channel. These results indicate that the AR6 domain contributes to TRPV6-mediated maintenance of intracellular Ca2+ concentrations, and that this region could play a novel role in regulating the activity of TRPV6 Ca2+-selective channels.

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.

High-resolution structures of transient receptor potential vanilloid channels: Unveiling a functionally diverse group of ion channels

Transient receptor potential vanilloid (TRPV) channels are part of the superfamily of TRP ion channels and play important roles in widespread physiological processes including both neuronal and non‐neuronal pathways. Various diseases such as skeletal abnormalities, chronic pain, and cancer are associated with dysfunction of a TRPV channel. In order to obtain full understanding of disease pathogenesis and create opportunities for therapeutic intervention, it is essential to unravel how these channels function at a molecular level. In the past decade, incredible progress has been made in biochemical sample preparation of large membrane proteins and structural biology techniques, including cryo‐electron microscopy. This has resulted in high resolution structures of all TRPV channels, which has provided novel insights into the molecular mechanisms of channel gating and regulation that will be summarized in this review.

Post-mortem histology in transient receptor potential cation channel subfamily V member 6 (TRPV6) under-mineralising skeletal dysplasia suggests postnatal skeletal recovery: a case report

Background

The calcium-selective channel TRPV6 (transient receptor potential cation channel subfamily V member 6) is crucial for maternal-fetal calcium transport across the placenta. TRPV6 mutations have recently been associated with an antenatally severe under-mineralising skeletal dysplasia accompanied by postnatal biochemical abnormalities. This is the first post-mortem report in a patient with TRPV6 skeletal dysplasia.

Case presentation

The female infant had severe antenatal and postnatal skeletal abnormalities by 20 weeks gestation and was ventilator-dependent from birth. These skeletal abnormalities were apparent at an earlier gestational age than in previous reported cases and a more severe clinical course ensued. Biochemical and skeletal abnormalities, including bone density, improved postnatally but cardiac arrest at 4 months of age led to withdrawal of intensive care. Compound heterozygous TRPV6 variants (c.1978G > C p.(Gly660Arg) and c.1528C > T p.(Arg510Ter)) were identified on exome sequencing. Post-mortem identified skeletal abnormalities but no specific abnormalities in other organ systems. No placental pathology was found, multi-organ histological features reflected prolonged intensive care only. Post-mortem macroscopic examination indicated reduced thoracic size and short, pale and pliable ribs. Histological examination identified reduced number of trabeculae in the diaphyses (away from the growth plates), whereas metaphyses showed adequate mineralisation and normal number of trabeculae, but with slightly enlarged reactive chondrocytes, indicating post-natal skeletal growth recovery. Post-mortem radiological findings demonstrated improved bone density, improved rib width, healed fractures, although ribs were still shorter than normal. Long bones (especially humerus and femur) had improved from initial poorly defined metaphyses and reduced bone density to sharply defined metaphyses, prominent growth restart lines in distal diaphyses and bone-in-bone appearance along diaphyses.

Conclusions

This case provide bone histological confirmation that human skeletal development is compromised in the presence of TRPV6 pathogenic variants. Post-mortem findings were consistent with abnormal in utero skeletal mineralisation due to severe calcium deficit from compromised placental calcium transfer, followed by subsequent phenotypic improvement with adequate postnatal calcium availability. Significant skeletal recovery occurs in the early weeks of postnatal life in TRPV6 skeletal dysplasia.

Structure and function of the calcium-selective TRP channel TRPV6

Epithelial calcium channel TRPV6 is a member of the vanilloid subfamily of TRP channels that is permeable to cations and highly selective to Ca²⁺ ; it shows constitutive activity regulated negatively by Ca²⁺ and positively by phosphoinositol and cholesterol lipids. In this review, we describe the molecular structure of TRPV6 and discuss how its structural elements define its unique functional properties. High Ca²⁺ selectivity of TRPV6 originates from the narrow selectivity filter, where Ca²⁺ ions are directly coordinated by a ring of anionic aspartate side chains. Divalent cations Ca²⁺ and Ba²⁺ permeate TRPV6 pore according to the knock-off mechanism, while tight binding of Gd³⁺ to the aspartate ring blocks the channel and prevents Na⁺ from permeating the pore. The iris-like channel opening is accompanied by an α-to-π helical transition in the pore-lining transmembrane helix S6. As a result of this transition, the intracellular halves of the S6 helices bend and rotate by about 100 deg, exposing different residues to the channel pore in the open and closed states. Channel opening is also associated with changes in occupancy of the transmembrane domain lipid binding sites. The inhibitor 2-aminoethoxydiphenyl borate (2-APB) binds to TRPV6 in a pocket formed by the cytoplasmic half of the S1-S4 transmembrane helical bundle and shifts open-closed channel equilibrium towards the closed state by outcompeting lipids critical for activation. Ca²⁺ inhibits TRPV6 via binding to calmodulin (CaM), which mediates Ca²⁺ -dependent inactivation. The TRPV6-CaM complex exhibits 1:1 stoichiometry; one TRPV6 tetramer binds both CaM lobes, which adopt a distinct head-to-tail arrangement. The CaM C-terminal lobe plugs the channel through a unique cation-π interaction by inserting the side chain of lysine K115 into a tetra-tryptophan cage at the ion channel pore intracellular entrance. Recent studies of TRPV6 structure and function described in this review advance our understanding of the role of this channel in physiology and pathophysiology and inform new therapeutic design.

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.

TRPV6 and Cav1.3 Mediate Distal Small Intestine Calcium Absorption Before Weaning

BACKGROUND & AIMS

Intestinal Ca²⁺ absorption early in life is vital to achieving optimal bone mineralization. The molecular details of intestinal Ca²⁺ absorption have been defined in adults after peak bone mass is obtained, but they are largely unexplored during development. We sought to delineate the molecular details of transcellular Ca²⁺ absorption during this critical period.

METHODS

Expression of small intestinal and renal calcium transport genes was assessed by using quantitative polymerase chain reaction. Net calcium flux across small intestinal segments was measured in Ussing chambers, including after pharmacologic inhibition or genetic manipulation of TRPV6 or Ca_v1.3 calcium channels. Femurs were analyzed by using micro-computed tomography and histology.

RESULTS

Net TRPV6-mediated Ca²⁺ flux across the duodenum was absent in pre-weaned (P14) mice but present after weaning. In contrast, we found significant transcellular Ca²⁺ absorption in the jejunum at 2 weeks but not 2 months of age. Net jejunal Ca²⁺ absorption observed at P14 was not present in either Trpv6 mutant (D541A) mice or Ca_v1.3 knockout mice. We observed significant nifedipine-sensitive transcellular absorption across the ileum at P14 but not 2 months. Ca_v1.3 knockout pups exhibited delayed bone mineral accrual, compensatory nifedipine-insensitive Ca²⁺ absorption in the ileum, and increased expression of renal Ca²⁺ reabsorption mediators at P14. Moreover, weaning pups at 2 weeks reduced jejunal and ileal Ca_v1.3 expression.

CONCLUSIONS

We have detailed novel pathways contributing to transcellular Ca²⁺ transport across the distal small intestine of mice during development, highlighting the complexity of the multiple mechanisms involved in achieving a positive Ca²⁺ balance early in life.

Maternal Transient Receptor Potential Vanilloid 6 (Trpv6) Is Involved In Offspring Bone Development

Embryonic growth and bone development depend on placental Ca²⁺ transport across the feto-maternal barrier to supply minerals to the fetus. The individual factors and cellular mechanisms that regulate placental Ca²⁺ transfer, however, are only beginning to emerge. We find that the Ca²⁺ -selective transient receptor potential vanilloid 6 (TRPV6) channel is expressed in trophoblasts of the fetal labyrinth, in the yolk sac, and in the maternal part of the placenta. Lack of functional TRPV6 channels in the mother leads to a reduced Ca²⁺ content in both placenta and embryo. Ca²⁺ uptake in trophoblasts is impaired in the absence of Trpv6. Trpv6-deficient embryos are smaller, have a lower body weight, and shorter and less calcified femurs. The altered cortical bone microarchitecture persists in adulthood. We show that TRPV6's Ca²⁺ -conducting property causes this embryonic and bone phenotype. Our results show that TRPV6 is necessary for the Ca²⁺ uptake in trophoblasts and that TRPV6 deficiency in the placenta leads to reduced embryo growth, minor bone calcification, and impaired bone development. © 2019 American Society for Bone and Mineral Research.

TRPV6 Gene Mutation in a Dizygous Twin With Transient Neonatal Hyperparathyroidism

Maternal-fetal transport of calcium (Ca²⁺) is important for bone mineralization in fetal development. Insufficient Ca²⁺ transport causes transient neonatal hyperparathyroidism (TNHP). Transient receptor potential cation channel, subfamily V, member 6 (TRPV6), has been found to play an important role in the active transport of Ca²⁺ through the placenta. Recently, TRPV6 gene was found to be the gene responsible for TNHP with severe skeletal undermineralization. To date, only seven cases of TNHP caused by TRPV6 recessive mutations have been reported. We present a case of TNHP caused by TRPV6 gene mutations. A female newborn was hospitalized because of respiratory distress. Marked undermineralization of the skeleton was observed in X-ray imaging. Laboratory examination revealed markedly high PTH and absence of hypercalcemia along with vitamin D deficiency. Her twin brother presented with almost no symptoms. Maternal laboratory findings indicated normocalcemia, but vitamin D deficiency with a high PTH level for the lactation period was observed. We initially diagnosed the patient as having secondary hyperparathyroidism because of maternal vitamin D deficiency. Nevertheless, the reasons underlying the discordant clinical manifestations between the twin siblings remained unclear. Our analysis of TRPV6 gene clarified that the patient had compound heterozygote mutations, which were reported previously (p.Ile223Thr and p.Gly428Arg). Pathologic mutations in TRPV6 gene were not detected in the other sibling. The clinical symptoms in the patient were transient: they resolved during infancy. TNHP caused by TRPV6 gene mutations is a unique disease in terms of its transient pathology in utero and relief after birth.

TRPV6 compound heterozygous variants result in impaired placental calcium transport and severe undermineralization and dysplasia of the fetal skeleton

Transient receptor potential vanilloid 6 (TRPV6) functions in tetramer form for calcium transport. Until now, TRPV6 has not been linked with skeletal development disorders. An infant with antenatal onset thoracic insufficiency required significant ventilatory support. Skeletal survey showed generalized marked undermineralization, hypoplastic fractured ribs, metaphyseal fractures, and extensive periosteal reaction along femoral, tibial, and humeral diaphyses. Parathyroid hormone (PTH) elevation (53.4-101 pmol/L) initially suggested PTH signaling disorders. Progressively, biochemical normalization with radiological mineralization suggested recovery from in utero pathophysiology. Genomic testing was undertaken and in silico protein modeling of variants. No abnormalities in antenatal CGH array or UPD14 testing. Postnatal molecular genetic analysis found no causative variants in CASR, GNA11, APS21, or a 336 gene skeletal dysplasia panel investigated by whole exome sequencing. Trio exome analysis identified compound heterozygous TRPV6 likely pathogenic variants: novel maternally inherited missense variant, c.1978G > C p.(Gly660Arg), and paternally inherited nonsense variant, c.1528C > T p.(Arg510Ter), confirming recessive inheritance. p.(Gly660Arg) generates a large side chain protruding from the C-terminal hook into the interface with the adjacent TRPV6 subunit. In silico protein modeling suggests steric clashes between interface residues, decreased C-terminal hook, and TRPV6 tetramer stability. The p.(Gly660Arg) variant is predicted to result in profound loss of TRPV6 activity. This first case of a novel dysplasia features severe but improving perinatal abnormalities. The TRPV6 compound heterozygous variants appear likely to interfere with fetoplacental calcium transfer crucial for in utero skeletal development. Astute clinical interpretation of evolving perinatal abnormalities remains valuable in complex calcium and bone pathophysiology and informs exome sequencing interpretation.