Novel TRPV6 mutations in the spectrum of transient neonatal hyperparathyroidism

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.

Bidirectional Allosteric Coupling between PIP2 Binding and the Pore of the Oncochannel TRPV6

The epithelial ion channel TRPV6 plays a pivotal role in calcium homeostasis. Channel function is intricately regulated at different stages, involving the lipid phosphatidylinositol-4,5-bisphosphate (PIP2). Given that dysregulation of TRPV6 is associated with various diseases, including different types of cancer, there is a compelling need for its pharmacological targeting. Structural studies provide insights on how TRPV6 is affected by different inhibitors, with some binding to sites else occupied by lipids. These include the small molecule cis-22a, which, however, also binds to and thereby blocks the pore. By combining calcium imaging, electrophysiology and optogenetics, we identified residues within the pore and the lipid binding site that are relevant for regulation by cis-22a and PIP2 in a bidirectional manner. Yet, mutation of the cytosolic pore exit reduced inhibition by cis-22a but preserved sensitivity to PIP2 depletion. Our data underscore allosteric communication between the lipid binding site and the pore and vice versa for most sites along the pore.

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.

Genetic testing for familial hyperparathyroidism: clinical-genetic profile in a Mediterranean cohort

Background

Approximately 10% of primary hyperparathyroidism cases are hereditary, due to germline mutations in certain genes. Although clinically relevant, a systematized genetic diagnosis is missing due to a lack of firm evidence regarding individuals to test and which genes to evaluate.

Methods

A customized gene panel (AIP, AP2S1, CASR, CDC73, CDKN1A, CDKN1B, CDKN2B, CDKN2C, GCM2, GNA11, MEN1, PTH, RET, and TRPV6) was performed in 40 patients from the Mediterranean area with suspected familial hyperparathyroidism (≤45 years of age, family history, high-risk histology, associated tumour, multiglandular disease, or recurrent hyperparathyroidism). We aimed to determine the prevalence of germline variants in these patients, to clinically characterize the probands and their relatives, and to compare disease severity in carriers versus those with a negative genetic test.

Results

Germline variants were observed in 9/40 patients (22.5%): 2 previously unknown pathogenic/likely pathogenic variants of CDKN1B (related to MEN4), 1 novel variant of uncertain significance of CDKN2C, 4 variants of CASR (3 pathogenic/likely pathogenic variants and 1 variant of uncertain significance), and 2 novel variants of uncertain significance of TRPV6. Familial segregation studies allowed diagnosis and early treatment of PHPT in first-degree relatives of probands.

Conclusion

The observed prevalence of germline variants in the Mediterranean cohort under study was remarkable and slightly higher than that seen in other populations. Genetic screening for suspected familial hyperparathyroidism allows the early diagnosis and treatment of PHPT and other related comorbidities. We recommend genetic testing for patients with primary hyperparathyroidism who present with high-risk features.

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.

TRPV6 channel mediates alcohol-induced gut barrier dysfunction and systemic response

Intestinal epithelial tight junction disruption is a primary contributing factor in alcohol-associated endotoxemia, systemic inflammation, and multiple organ damage. Ethanol and acetaldehyde disrupt tight junctions by elevating intracellular Ca²⁺. Here we identify TRPV6, a Ca²⁺-permeable channel, as responsible for alcohol-induced elevation of intracellular Ca²⁺, intestinal barrier dysfunction, and systemic inflammation. Ethanol and acetaldehyde elicit TRPV6 ionic currents in Caco-2 cells. Studies in Caco-2 cell monolayers and mouse intestinal organoids show that TRPV6 deficiency or inhibition attenuates ethanol- and acetaldehyde-induced Ca²⁺ influx, tight junction disruption, and barrier dysfunction. Moreover, Trpv6^−/− mice are resistant to alcohol-induced intestinal barrier dysfunction. Photoaffinity labeling of 3-azibutanol identifies a histidine as a potential alcohol-binding site in TRPV6. The substitution of this histidine, and a nearby arginine, reduces ethanol-activated currents. Our findings reveal that TRPV6 is required for alcohol-induced gut barrier dysfunction and inflammation. Molecules that decrease TRPV6 function have the potential to attenuate alcohol-associated tissue injury.

Meena et al. show that the mechanism of alcohol-induced gut permeability, endotoxemia, and systemic inflammation requires the TRPV6 channel. They show that ethanol activates TRPV6, induces calcium influx, and disrupts intestinal epithelial tight junctions. Furthermore, specific histidine and arginine residues at the N terminus fine-tune the alcohol-induced activation of TRPV6.

TRPV6 Regulation by Cis-22a and Cholesterol

The highly calcium-selective transient receptor potential vanilloid-type channel TRPV6 is important for epithelial Ca²⁺ transport. Proper regulation of the inherently constitutively active TRPV6 channels is intricate in preserving Ca²⁺ homeostasis, whereby structural and functional data suggest that lipids hold an essential role. Altered expression levels or specific TRPV6 mutations may lead to diseases, hence, TRPV6 represents an interesting target for pharmacological modulation. Recent cryo-EM data identified that the specific TRPV6 blocker cis-22a binds, apart from the pore, to a site within the tetrameric channel that largely matches a lipid binding pocket, LBS-2. Therein, cis-22a may replace a lipid such as cholesterol that is bound in the open state. Based on site-directed mutagenesis and functional recordings, we identified and characterized a series of residues within LBS-2 that are essential for TRPV6 inhibition by cis-22a. Additionally, we investigated the modulatory potential of diverse cholesterol depletion efforts on TRPV6 activity. While LBS-2 mutants exhibited altered maximum currents, slow Ca²⁺-dependent inactivation (SCDI) as well as less inhibition by cis-22a, TRPV6 activity was resistant to cholesterol depletion. Hence, lipids other than cholesterol may predominate TRPV6 regulation when the channel is expressed in HEK293 cells.

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.

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.

The role of TRPV channels in osteoporosis

As the world's population ages, the treatment of osteoporosis is a major problem to be addressed. The cause of osteoporosis remains unclear. Ca²⁺ is not only an important component of bones but also plays a key role in osteoporosis treatment. Transient receptor potential vanilloid (TRPV) channels are one of the TRP channel families that is widely distributed in various organs, playing an important role in the physiological regulation of the human body. Bone formation and bone absorption may require Ca²⁺ transport via TRPV channels. It has been proven that the TRPV subtypes 1, 2, 4, 5, 6 (TRPV1, TRPV2, TRPV4, TRPV5, TRPV6) may affect bone metabolism balance through selective regulation of Ca²⁺. They significantly regulate osteoblast/osteoclast proliferation, differentiation and function. The purpose of this review is to explore the mechanisms of TRPV channels involved in regulation of the differentiation of osteoblasts and osteoclasts, as well as to discuss the latest developments in current researches, which may provide new clues and directions for an in-depth study of osteoporosis and other related bone metabolic diseases.

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.

Mapping the expression of transient receptor potential channels across murine placental development

Transient receptor potential (TRP) channels play prominent roles in ion homeostasis by their ability to control cation influx. Mouse placentation is governed by the processes of trophoblast proliferation, invasion, differentiation, and fusion, all of which require calcium signaling. Although certain TRP channels have been shown to contribute to maternal–fetal transport of magnesium and calcium, a role for TRP channels in specific trophoblast functions has been disregarded. Using qRT-PCR and in situ hybridisation, the spatio-temporal expression pattern of TRP channels in the mouse placenta across gestation (E10.5–E18.5) was assessed. Prominent expression was observed for Trpv2, Trpm6, and Trpm7. Calcium microfluorimetry in primary trophoblast cells isolated at E14.5 of gestation further revealed the functional activity of TRPV2 and TRPM7. Finally, comparing TRP channels expression in mouse trophoblast stem cells (mTSCs) and mouse embryonic stem cells (mESC) confirmed the specific expression of TRPV2 during placental development. Moreover, TRP channel expression was similar in mTSCs compared to primary trophoblasts and validate mTSC as a model to study TRP channels in placental development. Collectivity, our results identify a specific spatio-temporal TRP channel expression pattern in trophoblasts, suggesting a possible involvement in regulating the process of placentation.