TRP Channels of Islets

Role of TRP Channels in Metabolism-Related Diseases

Metabolic syndrome (MetS), with its high prevalence and significant impact on cardiovascular disease, poses a substantial threat to human health. The early identification of pathological abnormalities related to MetS and prevention of the risk of associated diseases is of paramount importance. Transient Receptor Potential (TRP) channels, a type of nonselective cation channel, are expressed in a variety of tissues and have been implicated in the onset and progression of numerous metabolism-related diseases. This study aims to review and discuss the expression and function of TRP channels in metabolism-related tissues and blood vessels, and to elucidate the interactions and mechanisms between TRP channels and metabolism-related diseases. A comprehensive literature search was conducted using keywords such as TRP channels, metabolic syndrome, pancreas, liver, oxidative stress, diabetes, hypertension, and atherosclerosis across various academic databases including PubMed, Google Scholar, Elsevier, Web of Science, and CNKI. Our review of the current research suggests that TRP channels may be involved in the development of metabolism-related diseases by regulating insulin secretion and release, lipid metabolism, vascular functional activity, oxidative stress, and inflammatory response. TRP channels, as nonselective cation channels, play pivotal roles in sensing various intra- and extracellular stimuli and regulating ion homeostasis by osmosis. They present potential new targets for the diagnosis or treatment of metabolism-related diseases.

Phosphate, calcium, and vitamin D signaling, transport, and metabolism in the endometria of cyclic ewes

BACKGROUND

Recent evidence suggests important roles for progesterone (P4) and interferon tau in the regulation of calcium, phosphate, and vitamin D signaling in the uteri of pregnant sheep. However, the effects of P4 and estradiol (E2), with respect to the expression of their receptors PGR and ESR1, respectively, in uterine epithelia on mineral signaling during the estrous cycle has not been investigated. Estrous cycles of mature Suffolk ewes were synchronized, prostaglandin F2α was administered, and ewes were observed for estrus (designated as Day 0) in the presence of vasectomized rams. On Days 1, 9, or 14 of the estrous cycle, hysterectomies were performed.

RESULTS

25-hydroxyvitamin D was more abundant in plasma from ewes on Day 14 than Day 1 (P < 0.05). Expression of fibroblast growth factor receptor 2 (FGFR2), a disintegrin and metalloprotease 17 (ADAM17), and parathyroid hormone-related protein (PTHrP) mRNAs was greater in endometria on Day 9 compared to Days 1 and 14 (P < 0.01). Similarly, expression of transient receptor potential cation channel subfamily V member 6 (TRPV6) mRNA was greater in endometria on Day 9 than Day 1 (P < 0.05). ATPase plasma membrane Ca²⁺ transporting 4 (ATP2B4) and S100 calcium binding protein G (S100G) mRNA expression was greater in endometria on Day 14 than on Days 1 and 9 (P < 0.01). In contrast, endometrial expression of vitamin D receptor (VDR) mRNA was lower on Days 9 and 14 than Day 1 (P < 0.01). Expression of klotho (KL) (P < 0.05) and cytochrome P450 family 24 subfamily A member 1 (CYP24) (P < 0.01) mRNAs was lower on Day 14 than Days 1 and 9. ADAM17, FGF23, CYP2R1, CYP27B1, KL, and VDR proteins immunolocalized to the uterine myometrium, blood vessels, and uterine luminal (LE), superficial glandular (sGE), and glandular (GE) epithelia. S100A9 protein was weakly expressed in the uterine myometrium, LE, sGE, and GE. Immunoreactivity of CYP2R1 and KL proteins in uterine LE and sGE was less on Day 1 than on Days 9 and 14. In contrast, S100G protein was expressed exclusively by GE, and immunoreactive S100G protein was less on Day 9. S100A12 protein localized to stromal cells of the uterine stratum spongiosum and blood vessels, but not by uterine epithelial cells.

CONCLUSION

Collectively, these results implicate E2, P4, and PGR in the regulation of phosphate, calcium, and vitamin D signaling in cyclic ewes.

TRP Channels as Molecular Targets to Relieve Endocrine-Related Diseases

Transient receptor potential (TRP) channels are polymodal channels capable of sensing environmental stimuli, which are widely expressed on the plasma membrane of cells and play an essential role in the physiological or pathological processes of cells as sensors. TRPs often form functional homo- or heterotetramers that act as cation channels to flow Na⁺ and Ca²⁺, change membrane potential and [Ca²⁺]_i (cytosolic [Ca²⁺]), and change protein expression levels, channel attributes, and regulatory factors. Under normal circumstances, various TRP channels respond to intracellular and extracellular stimuli such as temperature, pH, osmotic pressure, chemicals, cytokines, and cell damage and depletion of Ca²⁺ reserves. As cation transport channels and physical and chemical stimulation receptors, TRPs play an important role in regulating secretion, interfering with cell proliferation, and affecting neural activity in these glands and their adenocarcinoma cells. Many studies have proved that TRPs are widely distributed in the pancreas, adrenal gland, and other glands. This article reviews the specific regulatory mechanisms of various TRP channels in some common glands (pancreas, salivary gland, lacrimal gland, adrenal gland, mammary gland, gallbladder, and sweat gland).

Novel mineral regulatory pathways in ovine pregnancy: II. Calcium-binding proteins, calcium transporters, and vitamin D signaling

Mineralization of the fetal mammalian skeleton requires a hypercalcemic gradient across the placenta from mother to fetus. However, the mechanisms responsible for maintaining the placental transport of calcium remain poorly understood. This study aimed to identify calcium and vitamin D regulatory pathway components in ovine endometria and placentae across gestation. Suffolk ewes were bred with fertile rams upon detection of estrus (Day 0). On Days 9, 12, 17, 30, 70, 90, 110, and 125 of pregnancy (n=3-14/Day), ewes were euthanized and hysterectomized. Calcium abundance was influenced by gestational day in uterine flushings and allantoic fluid (P<0.05). The expression of S100G, S100A9, S100A12, ATP2B3, ATP2B4, TRPV5, TRPV6, CYP11A1, CYP2R1, CYP24, and VDR mRNAs known to be involved in calcium binding, calcium transport, and vitamin D metabolism were quantified by qPCR. Mediators of calcium and vitamin D signaling were expressed by Day 17 conceptus tissue, and endometria and placentae across gestation. Gestational day influenced the expression of S100G, S100A9, S100A12, TRPV6, VDR, and CYP24 mRNAs in endometria and placentae (P<0.05). Gestational day influenced endometrial expression of ATP2B3, and placental expression of TRPV5, ATP2B4, and CYP11A1 (P<0.05). VDR protein localized to the endoderm and trophectoderm (Day 17 conceptus) and was expressed in endometria and placentae throughout gestation. The observed spatiotemporal profile suggests a potential role of calcium and vitamin D in the establishment of pregnancy and regulation of fetal and placental growth, providing a platform for further mechanistic investigation.

Verapamil-induced ion channel and miRNA expression changes in rat testis and/or spermatozoa may be associated with male infertility

The effect of verapamil, a calcium channel blocker, on male fertility in terms of ion channel and miRNA gene expressions in testis/spermatozoa was evaluated in this study. Rats were divided into sham and verapamil groups (n = 15). Verapamil was performed orally for 60 days. Sperm parameters and levels of serum follicle-stimulating hormone (FSH), luteinising hormone (LH) and testosterone (T) hormones were analysed. Alterations of microRNA (miRNA) and ion channel gene expressions in spermatozoa/testis were detected by using qPCR. Verapamil treatment reduced sperm concentration. Increased serum FSH, LH and T hormone levels were detected. Upregulated transient receptor potential cation channel subfamily V member 5 (TRPV5) and potassium voltage-gated channel subfamily J member 11 (KCNJ11) gene expressions and downregulated miR-let-7b, miR-10a, miR-320 and miR-760 expressions were found in testis of verapamil group. However, upregulated anoctamin 1 (ANO1), ATP-binding cassette subfamily C member 9 (ABCC9), miR-27a and miR-130a expressions and downregulated miR-20a, miR-92a, miR-132, miR-320 and miR-760 expressions were detected in spermatozoa. In addition, these altered gene expressions were found to be associated with decreased sperm concentration. The results indicate that the changes in testicular and/or spermatozoal ion channels and miRNA expressions due to verapamil treatment may affect male fertility.

Molecular Regulations and Functions of the Transient Receptor Potential Channels of the Islets of Langerhans and Insulinoma Cells

Insulin secretion from the β-cells of the islets of Langerhans is triggered mainly by nutrients such as glucose, and incretin hormones such as glucagon-like peptide-1 (GLP-1). The mechanisms of the stimulus-secretion coupling involve the participation of the key enzymes that metabolize the nutrients, and numerous ion channels that mediate the electrical activity. Several members of the transient receptor potential (TRP) channels participate in the processes that mediate the electrical activities and Ca²⁺ oscillations in these cells. Human β-cells express TRPC1, TRPM2, TRPM3, TRPM4, TRPM7, TRPP1, TRPML1, and TRPML3 channels. Some of these channels have been reported to mediate background depolarizing currents, store-operated Ca²⁺ entry (SOCE), electrical activity, Ca²⁺ oscillations, gene transcription, cell-death, and insulin secretion in response to stimulation by glucose and GLP1. Different channels of the TRP family are regulated by one or more of the following mechanisms: activation of G protein-coupled receptors, the filling state of the endoplasmic reticulum Ca²⁺ store, heat, oxidative stress, or some second messengers. This review briefly compiles our current knowledge about the molecular mechanisms of regulations, and functions of the TRP channels in the β-cells, the α-cells, and some insulinoma cell lines.

Stimulus-Secretion Coupling in Beta-Cells: From Basic to Bedside

Insulin secretion in humans is usually induced by mixed meals, which upon ingestion, increase the plasma concentration of glucose, fatty acids, amino acids, and incretins like glucagon-like peptide 1. Beta-cells can stay in the off-mode, ready-mode or on-mode; the mode-switching being determined by the open state probability of the ATP-sensitive potassium channels, and the activity of enzymes like glucokinase, and glutamate dehydrogenase. Mitochondrial metabolism is critical for insulin secretion. A sound understanding of the intermediary metabolism, electrophysiology, and cell signaling is essential for comprehension of the entire spectrum of the stimulus-secretion coupling. Depolarization brought about by inhibition of the ATP sensitive potassium channel, together with the inward depolarizing currents through the transient receptor potential (TRP) channels, leads to electrical activities, opening of the voltage-gated calcium channels, and exocytosis of insulin. Calcium- and cAMP-signaling elicited by depolarization, and activation of G-protein-coupled receptors, including the free fatty acid receptors, are intricately connected in the form of networks at different levels. Activation of the glucagon-like peptide 1 receptor augments insulin secretion by amplifying calcium signals by calcium induced calcium release (CICR). In the treatment of type 2 diabetes, use of the sulfonylureas that act on the ATP sensitive potassium channel, damages the beta cells, which eventually fail; these drugs do not improve the cardiovascular outcomes. In contrast, drugs acting through the glucagon-like peptide-1 receptor protect the beta-cells, and improve cardiovascular outcomes. The use of the glucagon-like peptide 1 receptor agonists is increasing and that of sulfonylurea is decreasing. A better understanding of the stimulus-secretion coupling may lead to the discovery of other molecular targets for development of drugs for the prevention and treatment of type 2 diabetes.

Regulation of Membrane Calcium Transport Proteins by the Surrounding Lipid Environment

Calcium ions (Ca2+) are major messengers in cell signaling, impacting nearly every aspect of cellular life. Those signals are generated within a wide spatial and temporal range through a large variety of Ca2+ channels, pumps, and exchangers. More and more evidences suggest that Ca2+ exchanges are regulated by their surrounding lipid environment. In this review, we point out the technical challenges that are currently being overcome and those that still need to be defeated to analyze the Ca2+ transport protein-lipid interactions. We then provide evidences for the modulation of Ca2+ transport proteins by lipids, including cholesterol, acidic phospholipids, sphingolipids, and their metabolites. We also integrate documented mechanisms involved in the regulation of Ca2+ transport proteins by the lipid environment. Those include: (i) Direct interaction inside the protein with non-annular lipids; (ii) close interaction with the first shell of annular lipids; (iii) regulation of membrane biophysical properties (e.g., membrane lipid packing, thickness, and curvature) directly around the protein through annular lipids; and (iv) gathering and downstream signaling of several proteins inside lipid domains. We finally discuss recent reports supporting the related alteration of Ca2+ and lipids in different pathophysiological events and the possibility to target lipids in Ca2+-related diseases.

New Insights into Beta-Cell GLP-1 Receptor and cAMP Signaling

Harnessing the translational potential of the GLP-1/GLP-1R system in pancreatic beta cells has led to the development of established GLP-1R-based therapies for the long-term preservation of beta cell function. In this review, we discuss recent advances in the current research on the GLP-1/GLP-1R system in beta cells, including the regulation of signaling by endocytic trafficking as well as the application of concepts such as signal bias, allosteric modulation, dual agonism, polymorphic receptor variants, spatial compartmentalization of cAMP signaling and new downstream signaling targets involved in the control of beta cell function.

Validation of impaired Transient Receptor Potential Melastatin 3 ion channel activity in natural killer cells from Chronic Fatigue Syndrome/ Myalgic Encephalomyelitis patients

Background

Chronic Fatigue Syndrome/ Myalgic Encephalomyelitis (CFS/ME) is a complex multifactorial disorder of unknown cause having multi-system manifestations. Although the aetiology of CFS/ME remains elusive, immunological dysfunction and more particularly reduced cytotoxic activity in natural killer (NK) cells is the most consistent laboratory finding. The Transient Receptor Potential (TRP) superfamily of cation channels play a pivotal role in the pathophysiology of immune diseases and are therefore potential therapeutic targets. We have previously identified single nucleotide polymorphisms in TRP genes in peripheral NK cells from CFS/ME patients. We have also described biochemical pathway changes and calcium signaling perturbations in NK cells from CFS/ME patients. Notably, we have previously reported a decrease of TRP cation channel subfamily melastatin member 3 (TRPM3) function in NK cells isolated from CFS/ME patients compared with healthy controls after modulation with pregnenolone sulfate and ononetin using a patch-clamp technique. In the present study, we aim to confirm the previous results describing an impaired TRPM3 activity in a new cohort of CFS/ME patients using a whole cell patch-clamp technique after modulation with reversible TRPM3 agonists, pregnenolone sulfate and nifedipine, and an effective TRPM3 antagonist, ononetin. Indeed, no formal research has commented on using pregnenolone sulfate or nifedipine to treat CFS/ME patients while there is evidence that clinicians prescribe calcium channel blockers to improve different symptoms.

Methods

Whole-cell patch-clamp technique was used to measure TRPM3 activity in isolated NK cells from twelve age- and sex-matched healthy controls and CFS/ME patients, after activation with pregnenolone sulfate and nifedipine and inhibition with ononetin.

Results

We confirmed a significant reduction in amplitude of TRPM3 currents after pregnenolone sulfate stimulation in isolated NK cells from another cohort of CFS/ME patients compared with healthy controls. The pregnenolone sulfate-evoked ionic currents through TRPM3 channels were again significantly modulated by ononetin in isolated NK cells from healthy controls compared with CFS/ME patients. In addition, we used nifedipine, another reversible TRPM3 agonist to support the previous findings and found similar results confirming a significant loss of the TRPM3 channel activity in CFS/ME patients.

Conclusions

Impaired TRPM3 activity was validated in NK cells isolated from CFS/ME patients using different pharmacological tools and whole-cell patch-clamp technique as the gold standard for ion channel research. This investigation further helps to establish TRPM3 channels as a prognostic marker and/ or a potential therapeutic target for CFS/ME.

TRPV2 channels mediate insulin secretion induced by cell swelling in mouse pancreatic β-cells

β-Cell swelling induces membrane depolarization, which has been suggested to be caused at least partly by the activation of cation channels. Here, we show the identification of the cation channels. In isolated mouse pancreatic β-cells, the exposure to 30% hypotonic solution elicited an increase in cytosolic Ca²⁺ concentration ([Ca²⁺]_c). The [Ca²⁺]_c elevation was partially inhibited by ruthenium red, a blocker of several Ca²⁺-permeable channels including transient receptor potential vanilloid receptors [transient receptor potential cation channel subfamily V (TRPV)], and by nicardipine, but not by the depletion of intracellular Ca²⁺ stores with thapsigargin and caffeine. The hypotonic stimulation also increased insulin secretion from isolated mouse islets, which was significantly suppressed by ruthenium red. Expression of TRPV2 and TRPV4 was confirmed in mouse pancreatic islets and the MIN6 β-cell line by RT-PCR, Western blot, and immunohistochemical analyses. However, neither 4α-phorbol 12,13-didecanoate nor GSK1016790A, TRPV4 activators, showed any apparent effect on [Ca²⁺]_c in isolated mouse β-cells or in MIN6 cells. In contrast, probenecid, a TRPV2 activator, induced an increase in [Ca²⁺]_c in MIN6 cells, which was attenuated by ruthenium red. Moreover, the [Ca²⁺]_c elevation induced by 30% hypotonic stimulation was significantly reduced by knockdown of TRPV2 with siRNA and by tranilast, a TRPV2 inhibitor. The knockdown of TRPV2 also decreased insulin secretion induced by the hypotonic stimulation. In addition, glucose-stimulated insulin secretion was also significantly reduced in the TRPV2-knockdown MIN6 cells. These results suggest that osmotic cell swelling activates TRPV2 in mouse β-cells, thereby causing membrane depolarization and subsequent activation of voltage-dependent Ca²⁺ channels and insulin secretion.

Emerging role of testosterone in pancreatic β-cell function and insulin secretion

One of the most sexually dimorphic aspects of metabolic regulation is the bidirectional modulation of glucose homeostasis by testosterone in male and females. Severe testosterone deficiency predisposes men to type 2 diabetes (T2D), while in contrast, androgen excess predisposes women to hyperglycemia. The role of androgen deficiency and excess in promoting visceral obesity and insulin resistance in men and women respectively is well established. However, although it is established that hyperglycemia requires β cell dysfunction to develop, the role of testosterone in β cell function is less understood. This review discusses recent evidence that the androgen receptor (AR) is present in male and female β cells. In males, testosterone action on AR in β cells enhances glucose-stimulated insulin secretion by potentiating the insulinotropic action of glucagon-like peptide-1. In females, excess testosterone action via AR in β cells promotes insulin hypersecretion leading to oxidative injury, which in turn predisposes to T2D.

Catechol estrogens stimulate insulin secretion in pancreatic β-cells via activation of the transient receptor potential A1 (TRPA1) channel

Estrogen hormones play an important role in controlling glucose homeostasis and pancreatic β-cell function. Despite the significance of estrogen hormones for regulation of glucose metabolism, little is known about the roles of endogenous estrogen metabolites in modulating pancreatic β-cell function. In this study, we evaluated the effects of major natural estrogen metabolites, catechol estrogens, on insulin secretion in pancreatic β-cells. We show that catechol estrogens, hydroxylated at positions C2 and C4 of the steroid A ring, rapidly potentiated glucose-induced insulin secretion via a nongenomic mechanism. 2-Hydroxyestrone, the most abundant endogenous estrogen metabolite, was more efficacious in stimulating insulin secretion than any other tested catechol estrogens. In insulin-secreting cells, catechol estrogens produced rapid activation of calcium influx and elevation in cytosolic free calcium. Catechol estrogens also generated sustained elevations in cytosolic free calcium and evoked inward ion current in HEK293 cells expressing the transient receptor potential A1 (TRPA1) cation channel. Calcium influx and insulin secretion stimulated by estrogen metabolites were dependent on the TRPA1 activity and inhibited with the channel-specific pharmacological antagonists or the siRNA. Our results suggest the role of estrogen metabolism in a direct regulation of TRPA1 activity with potential implications for metabolic diseases.

Androgen receptor-deficient islet β-cells exhibit alteration in genetic markers of insulin secretion and inflammation. A transcriptome analysis in the male mouse

AIMS

Testosterone action is mediated via the androgen receptor (AR). We have reported that male mice lacking AR selectively in β-cells (βARKO^-/y) develop decreased glucose-stimulated insulin secretion (GSIS), producing glucose intolerance. We showed that testosterone action on AR in β-cells amplifies the insulinotropic action of GLP-1 on its receptor via a cAMP-dependent protein kinase-A pathway.

METHODS

To investigate AR-dependent gene networks in β-cells, we performed a high throughput whole transcriptome sequencing (RNA-Seq) in islets from male βARKO^-/y and control mice.

RESULTS

We identified 214 differentially expressed genes (DEGs) (158 up- and 56 down-regulated) with a false discovery rate (FDR) < 0.05 and a fold change (FC) > 2. Our analysis of individual transcripts revealed alterations in β-cell genes involved in cellular inflammation/stress and insulin secretion. Based on 312 DEGs with an FDR < 0.05, the pathway analysis revealed 23 significantly enriched pathways, including cytokine-cytokine receptor interaction, Jak-STAT signaling, insulin signaling, MAPK signaling, type 2 diabetes (T2D) and pancreatic secretion. The gene ontology analysis confirmed the results of the individual DEGs and the pathway analysis in showing enriched biological processes encompassing inflammation, ion transport, exocytosis and insulin secretion.

CONCLUSIONS

AR-deficient islets exhibit altered expression of genes involved in inflammation and insulin secretion demonstrating the importance of androgen action in β-cell health in the male with implications for T2D development in men.

Expression of Transient Receptor Potential Channels in the Purified Human Pancreatic β-Cells

OBJECTIVE

Members of the transient receptor potential (TRP) channels are involved in mediating the electrical excitability and stimulus-secretion coupling in the pancreatic β-cells. The expression and the relative abundance of different TRP channels in the human β-cells are unknown. The objective of this study was to examine the expression of the TRP channels and their relative abundance in the human β-cell.

METHODS

RNA sequencing data obtained from human islets, fluorescence-activated cell sorting-purified human β-cell and human pancreatic acinar cells were analyzed. Gene counts and fragments per kilobase per million mapped reads were obtained.

RESULTS

Among the TRPC family only the TRPC1 was expressed in the human β-cell. TRPV1 channels were not expressed in the human β-cells. Among the TRPM family, TRPM4, TRPM7, TRPM2, and TRPM3 were expressed in the human β-cell. Of the remaining TRP channels, TRPP2, TRPML1, and TRPML3 were expressed in these cells.

CONCLUSIONS

By analyzing the RNA sequencing data, we have detected for the first time the TRP channels that are expressed in the purified human β-cells, in comparison to the other relevant pancreatic cell types. Our study provides an opportunity to focus on these TRP channels for a better understanding of the electrophysiology and stimulus-secretion coupling in these cells.

A TRPM4 Inhibitor 9-Phenanthrol Inhibits Glucose- and Glucagon-Like Peptide 1-Induced Insulin Secretion from Rat Islets of Langerhans

Pancreatic β-cells express several ion channels of the transient receptor potential family, which play important roles in mediating the stimulus-secretion coupling. One of these channels, the TRPM4 is a Ca²⁺-activated monovalent cation channel. This channel is inhibited by 9-phenanthrol, which also inhibits the TMEM16a Cl^- channel, and activates the Ca²⁺-activated K⁺ channel, K_ca3.1. The net effects of ion-channel modulation by 9-phenantherol on the insulin secretion remain unclear. We tested the effects of 9-phenanthrol on glucose- and GLP-1-induced insulin secretion from isolated rat islets in static incubations. When applied to the islets in the presence of 3.3 mM glucose, 9-phenanthrol caused a small increase in insulin secretion (~7% of the insulin secretion stimulated by 10 mM glucose). 10 μM 9-phenanthrol did not inhibit glucose- or GLP-1-induced insulin secretion. 20 μM and 30 μM 9-phenanthrol inhibited glucose-induced insulin secretion by ~80% and ~85%, respectively. Inhibition of the GLP-1-induced insulin secretion by 20 μM and 30 μM 9-phenanthrol was 65% and 94%, respectively. Our study shows that the major effect of 9-phenanthrol on the islets is a strong inhibition of insulin secretion, and we speculate that compounds related to 9-phenanthrol may be potentially useful in treating the pancreatogenous hyperinsulinemic hypoglycemia syndromes.

Stromal Interaction Molecule 1 (STIM1) Regulates ATP-sensitive Potassium (KATP) and Store-operated Ca2+ Channels in MIN6 β-Cells

Stromal interaction molecule 1 (STIM1) regulates store-operated Ca²⁺ entry (SOCE) and other ion channels either as an endoplasmic reticulum Ca²⁺-sensing protein or when present in the plasma membrane. However, the role of STIM1 in insulin-secreting β-cells is unresolved. We report that lowering expression of STIM1, the gene that encodes STIM1, in insulin-secreting MIN6 β-cells with RNA interference inhibits SOCE and ATP-sensitive K⁺ (K_ATP) channel activation. The effects of STIM1 knockdown were reversed by transduction of MIN6 cells with an adenovirus gene shuttle vector that expressed human STIM1 Immunoprecipitation studies revealed that STIM1 binds to nucleotide binding fold-1 (NBF1) of the sulfonylurea receptor 1 (SUR1) subunit of the K_ATP channel. Binding of STIM1 to SUR1 was enhanced by poly-lysine. Our data indicate that SOCE and K_ATP channel activity are regulated by STIM1. This suggests that STIM1 is a multifunctional signaling effector that participates in the control of membrane excitability and Ca²⁺ signaling events in β-cells.

Glucose and GTP-binding protein-coupled receptor cooperatively regulate transient receptor potential-channels to stimulate insulin secretion [Review]

In pancreatic β-cells, glucose-induced closure of the ATP-sensitive K⁺ (KATP) channel is an initial process triggering glucose-stimulated insulin secretion (GSIS). This KATP-channel dependent pathway has been believed to be a central mechanism for GSIS. However, since the resting membrane potential of cells is determined by the balance of the net result of current amplitudes in outward and inward directions, it must be taken into consideration that not only KATP channel inhibition but also inward current via the basal opening of non-selective cation channels (NSCCs) plays a crucial role in membrane potential regulation. The basal activity of NSCCs is essential to effectively evoke depolarization in concert with KATP channel closure that is dependent on glucose metabolism. The present study summarizes recent findings regarding the roles of NSCCs in GSIS and GTP-binding protein coupled receptor-(GPCR) operated potentiation of GSIS.

Ocular transient receptor potential channel function in health and disease

Transient receptor potential (TRP) channels sense and transduce environmental stimuli into Ca(2+) transients that in turn induce responses essential for cell function and adaptation. These non-selective channels with variable Ca(2+) selectivity are grouped into seven different subfamilies containing 28 subtypes based on differences in amino acid sequence homology. Many of these subtypes are expressed in the eye on both neuronal and non-neuronal cells where they affect a host of stress-induced regulatory responses essential for normal vision maintenance. This article reviews our current knowledge about the expression, function and regulation of TRPs in different eye tissues. We also describe how under certain conditions TRP activation can induce responses that are maladaptive to ocular function. Furthermore, the possibility of an association between TRP mutations and disease is considered. These findings contribute to evidence suggesting that drug targeting TRP channels may be of therapeutic benefit in a clinical setting. We point out issues that must be more extensively addressed before it will be possible to decide with certainty that this is a realistic endeavor. Another possible upshot of future studies is that disease process progression can be better evaluated by profiling changes in tissue specific functional TRP subtype activity as well as their gene and protein expression.

Polymodal roles of transient receptor potential channels in the control of ocular function

Maintenance of intracellular Ca²⁺ levels at orders of magnitude below those in the extracellular environment is a requisite for preserving cell viability. Membrane channels contribute to such control through modulating their time-dependent opening and closing behaviour. Such regulation requires Ca²⁺ to serve as a second messenger mediating receptor control of numerous life-sustaining responses. Transient receptor potential (TRP) channels signal transduce a wide variety of different sensory stimuli to induce responses modulating cellular function. These channels are non-selective cation channels with variable Ca²⁺ selectivity having extensive sequence homology. They constitute a superfamily made up of 28 different members that are subdivided into 7 different subfamilies based on differences in sequence homology. Some of these TRP channel isotypes are expressed in the eye and localized to both neuronal and non-neuronal cell membranes. Their activation generates intracellular Ca²⁺ transients and other downstream-linked signalling events that affect numerous responses required for visual function. As there is an association between changes in functional TRP expression in various ocular diseases, there are efforts underway to determine if these channels can be used as drug targets to reverse declines in ocular function. We review here our current knowledge about the expression, function and regulation of TRPs in different eye tissues in health and disease. Furthermore, some of the remaining hurdles are described to developing safe and efficacious TRP channel modulators for use in a clinical setting.

Activation and inhibition of transient receptor potential TRPM3-induced gene transcription

BACKGROUND AND PURPOSE

Transient receptor potential-3 (TRPM3) channels function as Ca2+ permeable cation channels. While the natural ligands for these channels are still unknown, several compounds have been described that either activate or inhibit TRPM3 channel activity. experimental approach: We assessed TRPM3-mediated gene transcription, which relies on the induction of intracellular signalling to the nucleus following activation of TRPM3 channels. Activator protein-1 (AP-1) and Egr-1-responsive reporter genes were integrated into the chromatin of the cells. This strategy enabled us to analyse gene transcription of the AP-1 and Egr-1-responsive reporter genes that were packed into an ordered chromatin structure.

KEY RESULTS

The neurosteroid pregnenolone sulfate strikingly up-regulated AP-1 and Egr-1 transcriptional activity, while nifedipine and D-erythro-sphingosine, also putative activators of TRPM3 channels, exhibited either no or TRPM3-independent effects on gene transcription. In addition, pregnenolone sulfate robustly enhanced the transcriptional activation potential of the ternary complex factor Elk-1. Pregnenolone sulfate-induced activation of gene transcription was blocked by treatment with mefenamic acid and, to a lesser extent, by the polyphenol naringenin. In contrast, progesterone, pregnenolone and rosiglitazone reduced AP-1 activity in the cells, but had no inhibitory effect on Egr-1 activity in pregnenolone sulfate-stimulated cells.

CONCLUSION AND IMPLICATIONS

Pregnenolone sulfate is a powerful activator of TRPM3-mediated gene transcription, while transcription is completely inhibited by mefenamic acid in cells expressing activated TRPM3 channels. Both compounds are valuable tools for further investigating the biological functions of TRPM3 channels.

Autocrine activation of P2Y1 receptors couples Ca (2+) influx to Ca (2+) release in human pancreatic beta cells

AIMS/HYPOTHESIS

There is evidence that ATP acts as an autocrine signal in beta cells but the receptors and pathways involved are incompletely understood. Here we investigate the receptor subtype(s) and mechanism(s) mediating the effects of ATP on human beta cells.

METHODS

We examined the effects of purinergic agonists and antagonists on membrane potential, membrane currents, intracellular Ca(2+) ([Ca(2+)]i) and insulin secretion in human beta cells.

RESULTS

Extracellular application of ATP evoked small inward currents (3.4 ± 0.7 pA) accompanied by depolarisation of the membrane potential (by 14.4 ± 2.4 mV) and stimulation of electrical activity at 6 mmol/l glucose. ATP increased [Ca(2+)]i by stimulating Ca(2+) influx and evoking Ca(2+) release via InsP3-receptors in the endoplasmic reticulum (ER). ATP-evoked Ca(2+) release was sufficient to trigger exocytosis in cells voltage-clamped at -70 mV. All effects of ATP were mimicked by the P2Y(1/12/13) agonist ADP and the P2Y1 agonist MRS-2365, whereas the P2X(1/3) agonist α,β-methyleneadenosine-5-triphosphate only had a small effect. The P2Y1 antagonists MRS-2279 and MRS-2500 hyperpolarised glucose-stimulated beta cells and lowered [Ca(2+)]i in the absence of exogenously added ATP and inhibited glucose-induced insulin secretion by 35%. In voltage-clamped cells subjected to action potential-like stimulation, MRS-2279 decreased [Ca(2+)]i and exocytosis without affecting Ca(2+) influx.

CONCLUSIONS/INTERPRETATION

These data demonstrate that ATP acts as a positive autocrine signal in human beta cells by activating P2Y1 receptors, stimulating electrical activity and coupling Ca(2+) influx to Ca(2+) release from ER stores.

Expression, signaling and function of Egr transcription factors in pancreatic β-cells and insulin-responsive tissues

Egr-1 and the related zinc finger transcription factors Egr-2, Egr-3, and Egr-4 are stimulated by many extracellular signaling molecules and represent a convergence point for intracellular signaling cascades. Egr-1 expression is induced in insulinoma cells and pancreatic β-cells following stimulation with either glucose, or pregnenolone sulfate. Moreover, stimulation of Gαq and Gαs-coupled receptors enhances EGR-1 gene transcription. Functional studies revealed that Egr transcription factors control insulin biosynthesis via regulation of Pdx-1 expression. Glucose homeostasis and pancreatic islet size are regulated by Egr transcription factors, indicating that these proteins control central physiological parameters regulated by pancreatic β-cells. In addition, Egr-1 is an integral part of the insulin receptor signaling cascade in insulin-responsive tissues and influences insulin resistance.

Role of transient receptor potential melastatin-like subtype 5 channel in insulin secretion from rat β-cells

OBJECTIVE

Several studies have reported that the transient receptor potential melastatin-like subtype 5 (TRPM5) channel, a Ca(2+)-activated monovalent cation channel, is involved in the stimulus-secretion coupling in the mouse pancreatic β-cells. We have studied the role of the TRPM5 channel in regulating insulin secretion and cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)) in the rat β-cells by using triphenylphosphine oxide, a selective inhibitor of the channel.

METHODS

Insulin secretion from islets from Sprague-Dawley rats was measured in batch incubations. Cytoplasmic free Ca(2+) concentration was measured from single β-cells by fura-2-based microfluorometry.

RESULTS

Triphenylphosphine oxide did not alter insulin secretion and [Ca(2+)](i) response triggered by KCl or fructose. It inhibited insulin secretion in response to glucose, L-arginine, and glucagon-like peptide 1. It also inhibited glucose-induced insulin secretion by mechanisms that are independent of the adenosine triphosphate-sensitive potassium channels and [Ca(2+)](i) increase.

CONCLUSIONS

Our results suggest that in the rat islets, TRPM5 is involved in mediating insulin secretion by glucose and l-arginine and in potentiating the glucose-induced insulin secretion by glucagon-like peptide 1.

Role for the TRPV1 channel in insulin secretion from pancreatic beta cells

Transient receptor potential channels have been put forward as regulators of insulin secretion. A role for the TRPV1 ion channel in insulin secretion has been suggested in pancreatic beta cell lines. We explored whether TRPV1 is functionally expressed in RINm5F and primary beta cells from neonate and adult rats. We examined if capsaicin could activate cationic non-selective currents. Our results show that TRPV1 channels are not functional in insulin-secreting cells, since capsaicin did not produce current activation, not even under culture conditions known to induce the expression of other ion channels in these cells. Although TRPV1 channels seem to be irrelevant for the physiology of isolated beta cells, they may play a role in glucose homeostasis acting through the nerve fibers that regulate islet function. At the physiological level, we observed that Trpv1 (-/-) mice presented lower fasting insulin levels than their wild-type littermates, however, we did not find differences between these experimental groups nor in the glucose tolerance test or in the insulin secretion. However, we did find that the Trpv1 (-/-) mice exhibited a higher insulin sensitivity compared to their wild-type counterparts. Our results demonstrate that TRPV1 does not contribute to glucose-induced insulin secretion in beta cells as was previously thought, but it is possible that it may control insulin sensitivity.

The Concise Guide to PHARMACOLOGY 2013/14: ion channels

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. Ion channels are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

The TRPA1 channel and oral hypoglycemic agents: is there complicity in β-cell exhaustion?

Diabetes mellitus type 2 (DM2) results from the combination of insulin unresponsiveness in target tissues and the failure of pancreatic β cells to secrete enough insulin. (1) It is a highly prevalent chronic disease that is aggravated with time, leading to major complications, such as cardiovascular disease and peripheral and ocular neuropathies. (2) Interestingly, therapies to improve glucose homeostasis in diabetic patients usually involve the use of glibenclamide, an oral hypoglycemic drug that blocks ATP-sensitive K(+) channels (KATP), (3)(,) (4) forcing β cells to release more insulin to overcome peripheral insulin resistance. However, sulfonylureas are ineffective for long-term treatments and ultimately result in the administration of insulin to control glucose levels. (5) The mechanisms underlying β-cell failure to respond effectively with glibenclamide after long-term treatments still needs clarification. A recent study demonstrating that this drug activates TRPA1, (6) a member of the Transient Receptor Potential (TRP) family of ion channels and a functional protein in insulin secreting cells, (7)(,) (8) has highlighted a possible role for TRPA1 as a potential mediator of sulfonylurea-induced toxicity.

Enhanced insulin clearance in mice lacking TRPM8 channels

Blood glucose concentration is tightly regulated by the rate of insulin secretion and clearance, a process partially controlled by sensory neurons serving as metabolic sensors in relevant tissues. The activity of these neurons is regulated by the products of metabolism which regulate transmitter release, and recent evidence suggests that neuronally expressed ion channels of the transient receptor potential (TRP) family function in this critical process. Here, we report the novel finding that the cold and menthol-gated channel TRPM8 is necessary for proper insulin homeostasis. Mice lacking TRPM8 respond normally to a glucose challenge while exhibiting prolonged hypoglycemia in response to insulin. Additionally, Trpm8-/- mice have increased rates of insulin clearance compared with wild-type animals and increased expression of insulin-degrading enzyme in the liver. TRPM8 channels are not expressed in the liver, but TRPM8-expressing sensory afferents innervate the hepatic portal vein, suggesting a TRPM8-mediated neuronal control of liver insulin clearance. These results demonstrate that TRPM8 is a novel regulator of serum insulin and support the role of sensory innervation in metabolic homeostasis.

The TRPM2 ion channel, an oxidative stress and metabolic sensor regulating innate immunity and inflammation

TRPM2 (transient receptor potential melastatin 2) is the unique fusion of a Ca(2+)-permeable pore with an enzymatic domain that binds the NAD(+)-metabolite ADP-ribose (ADPR), resulting in channel opening. ADPR formation is a metabolic corollary of cellular stress, but can also be elicited enzymatically through NAD glycohydrolases like CD38. TRPM2 thus functions as a metabolic and oxidative stress sensor and translates this information into ion fluxes that can affect Ca(2+) signaling and the membrane potential. TRPM2 is strongly represented in immune cells of the phagocytic lineage, themselves professional generators of oxidants. The recent characterization of TRPM2-deficient mouse models has revealed the involvement of this channel in various aspects of immunity. Monocytes lacking TRPM2 show reduced production of the CXCL2 chemokine, resulting in diminished neutrophilic influx to the colon in chemically induced colitis, and thus protection against tissue ulceration in TRPM2(-/-) mice. However, the insufficient production of proinflammatory cytokines leads to high morbidity and lethality of the TRPM2(-/-) mice following infection with the bacterial pathogen Listeria monocytogenes. In the context of endotoxin-induced pulmonary inflammation, TRPM2's absence was found to promote inflammation and ROS production. TRPM2 acts thereby as a negative feedback loop by interfering through membrane depolarization with ROS generation by NADPH oxidases. In dendritic cells, TRPM2 is a lysosomal Ca(2+)-release channel that promotes chemokine responsiveness and cell migration, which is reminiscent of CD38-mediated functions. The discovery of TRPM2 has unveiled an unsuspected signaling pathway and established ADPR as a novel second messenger. Understanding TRPM2's complex involvement in inflammation is crucial to evaluating the potential of manipulating TRPM2 activity and ADPR metabolism for therapeutic intervention.

Signal transduction via TRPM3 channels in pancreatic β-cells

Transient receptor potential melastatin 3 (TRPM3) channels are non-selective cation channels that are expressed in insulinoma cells and pancreatic β-cells. Stimulation of TRPM3 with the neurosteroid pregnenolone sulfate induces an intracellular signaling cascade, involving a rise in intracellular Ca(2)(+) concentration, activation of the protein kinases Raf and ERK, and a change in the gene expression pattern of the cells. In particular, biosynthesis of insulin is altered following activation of TRPM3 by pregnenolone sulfate. Moreover, a direct effect of TRPM3 stimulation on insulin secretion has been reported. The fact that stimulation of TRPM3 induces a signaling cascade that is very similar to the signaling cascade induced by glucose in β-cells suggests that TRPM3 may influence main functions of pancreatic β-cells. The view that TRPM3 represents an ionotropic steroid receptor of pancreatic β-cells linking insulin release with steroid hormone signaling is discussed.

Determination of beta-cell function: ion channel function in beta cells

For the regulation of beta-cell function ion channels are of outstanding importance. Beta cells are specialized to convert changes in blood glucose concentration to an adequate secretory response. To achieve this, nutrient-induced alterations of electrical activity are directly coupled to changes in insulin release. Consequently, determination and analysis of ion channel activity are important tools for the characterization of beta-cell (patho)physiology and for the investigation of drugs that influence insulin release. With implementation of the patch-clamp technique it has become possible to analyze ion currents in beta cells under various conditions (e.g., in intact cells or independent of cell metabolism, as whole-cell currents or on a single channel level). In addition, this method enables to combine ion current recordings with determination of membrane potential and exocytosis. This chapter introduces the basic principles of different patch-clamp configurations and focuses on experimental protocols for ion channel recordings in beta cells.

A cell permeable NPE caged ADP-ribose for studying TRPM2

Transient potential receptor melastatin-2 (TRPM2) is a non-selective Ca²⁺-permeable cation channel of the TRPM channel subfamily and is mainly activated by intracellular adenosine diphosphate ribose (ADPR). Here we synthesized a 1-(2-nitrophenyl)ethyl caged ADPR (NPE-ADPR) and found that uncaging of NPE-ADPR efficiently stimulated Ca²⁺, Mg²⁺, and Zn²⁺ influx in a concentration-dependent manner in intact human Jurkat T-lymphocytes. The cation influx was inhibited by inhibitors or knockdown of TRPM2. Likewise, uncaging of NPE-ADPR markedly induced cation entry in HEK 293 cells that overexpress TRPM2. As expected, high temperature increased the ability of the photolyzed NPE-ADPR to induce cation entry, whereas acidic pH inhibited. Moreover, the absence of extracellular Ca²⁺ significantly inhibited Mg²⁺ and Zn²⁺ influx after uncaging NPE-ADPR. On the other hand, the absence of extracellular Na⁺ or Mg²⁺ had no effect on photolyzed NPE-ADPR induced Ca²⁺ entry. Taken together, our results indicated that NPE-ADPR is a cell permeable ADPR analogue that is useful for studying TRPM2-mediated cation entry in intact cells.

Transient receptor potential melastatin 4 and cell death

Cell death proceeds by way of a variety of “cell death subroutines,” including several types of “apoptosis,” “regulated necrosis,” and others. “Accidental necrosis” due to profound adenosine triphosphate (ATP) depletion or oxidative stress is distinguished from regulated necrosis by the absence of death receptor signaling. However, both accidental and regulated necrosis have in common the process of “oncosis,” a physiological process characterized by Na⁺ influx and cell volume increase that, in necrotic cell death, is required to produce the characteristic features of membrane blebbing and membrane rupture. Here, we review emerging evidence that the monovalent cation channel, transient receptor potential melastatin 4 (TRPM4), is involved in the cell death process of oncosis. Potential involvement of TRPM4 in oncosis is suggested by the fact that the two principal regulators of TRPM4, intracellular ATP and Ca²⁺, are both altered during necrosis in the direction that causes TRPM4 channel opening. Under physiological conditions, activation of TRPM4 promotes Na⁺ influx and cell depolarization. Under pathological conditions, unchecked activation of TRPM4 leads to Na⁺ overload, cell volume increase, blebbing and cell membrane rupture, the latter constituting the irreversible end stage of necrosis. Emerging data indicate that TRPM4 plays a crucial role as end executioner in the accidental necrotic death of ATP-depleted or redox-challenged endothelial and epithelial cells, both in vitro and in vivo. Future studies will be needed to determine whether TRPM4 also plays a role in regulated necrosis and apoptosis.

Role of K(ATP) channels in β-cell resistance to oxidative stress

The importance of K(ATP) channels in stimulus-secretion coupling of β-cells is well established, although they are not indispensable for the maintenance of glycaemic control. This review article depicts a new role for K(ATP) channels by showing that genetic or pharmacological ablation of these channels protects β-cells against oxidative stress. Increased production of oxidants is a crucial factor in the pathogenesis of type 2 diabetes mellitus (T2DM). T2DM develops when β-cells can no longer compensate for the high demand of insulin resulting from excess fuel intake. Instead β-cells start to secrete less insulin and β-cell mass is diminished by apoptosis. Both, reduction of insulin secretion and β-cell mass induced by oxidative stress, are prevented by deletion or inhibition of K(ATP) channels. These findings may open up new insights into the early treatment of T2DM.

Expression of transient receptor potential ankyrin 1 (TRPA1) and its role in insulin release from rat pancreatic beta cells

Objective

Several transient receptor potential (TRP) channels are expressed in pancreatic beta cells and have been proposed to be involved in insulin secretion. However, the endogenous ligands for these channels are far from clear. Here, we demonstrate the expression of the transient receptor potential ankyrin 1 (TRPA1) ion channel in the pancreatic beta cells and its role in insulin release. TRPA1 is an attractive candidate for inducing insulin release because it is calcium permeable and is activated by molecules that are produced during oxidative glycolysis.

Methods

Immunohistochemistry, RT-PCR, and Western blot techniques were used to determine the expression of TRPA1 channel. Ca²⁺ fluorescence imaging and electrophysiology (voltage- and current-clamp) techniques were used to study the channel properties. TRPA1-mediated insulin release was determined using ELISA.

Results

TRPA1 is abundantly expressed in a rat pancreatic beta cell line and freshly isolated rat pancreatic beta cells, but not in pancreatic alpha cells. Activation of TRPA1 by allyl isothiocyanate (AITC), hydrogen peroxide (H₂O₂), 4-hydroxynonenal (4-HNE), and cyclopentenone prostaglandins (PGJ₂) and a novel agonist methylglyoxal (MG) induces membrane current, depolarization, and Ca²⁺ influx leading to generation of action potentials in a pancreatic beta cell line and primary cultured pancreatic beta cells. Activation of TRPA1 by agonists stimulates insulin release in pancreatic beta cells that can be inhibited by TRPA1 antagonists such as HC030031 or AP-18 and by RNA interference. TRPA1-mediated insulin release is also observed in conditions of voltage-gated Na⁺ and Ca²⁺ channel blockade as well as ATP sensitive potassium (K_ATP) channel activation.

Conclusions

We propose that endogenous and exogenous ligands of TRPA1 cause Ca²⁺ influx and induce basal insulin release and that TRPA1-mediated depolarization acts synergistically with K_ATP channel blockade to facilitate insulin release.

Insulin-secreting INS-1E cells express functional TRPV1 channels

We have studied whether functional TRPV1 channels exist in the INS-1E cells, a cell type used as a model for β-cells, and in primary β-cells from rat and human. The effects of the TRPV1 agonists capsaicin and AM404 on the intracellular free Ca (2+) concentration ([Ca (2+)]i) in the INS-1E cells were studied by fura-2 based microfluorometry. Capsaicin increased [Ca (2+)]i in a concentration-dependent manner, and the [Ca (2+)]i increase was dependent on extracellular Ca (2+). AM404 also increased [Ca (2+)]i in the INS-1E cells. Capsazepine, a specific antagonist of TRPV1, completely blocked the capsaicin- and AM404-induced [Ca (2+)]i increases. Capsaicin did not increase [Ca (2+)]i in the primary β-cells from rat and human. Whole cell patch clamp configuration was used to record currents across the plasma membrane in the INS-1E cells. Capsaicin elicited inward currents that were inhibited by capsazepine. Western blot analysis detected TRPV1 proteins in the INS-1E cells and the human islets. Immunohistochemistry was used to study the expression of TRPV1, but no TRPV1 protein immunoreactivity was detected in the human islet cells and the human insulinoma cells. We conclude that the INS-1E cells, but not the primary β-cells, express functional TRPV1 channels.