9-Phenanthrol inhibits recombinant and arterial myocyte TMEM16A channels

Uncovering the arrhythmogenic potential of TRPM4 activation in atrial-derived HL-1 cells using novel recording and numerical approaches

Aims

Transient receptor potential cation channel subfamily melastatin member 4 (TRPM4), a Ca2+-activated nonselective cation channel abundantly expressed in the heart, has been implicated in conduction block and other arrhythmic propensities associated with cardiac remodelling and injury. The present study aimed to quantitatively evaluate the arrhythmogenic potential of TRPM4.

Methods and results

Patch clamp and biochemical analyses were performed using expression system and an immortalized atrial cardiomyocyte cell line (HL-1), and numerical model simulation was employed. After rapid desensitization, robust reactivation of TRPM4 channels required high micromolar concentrations of Ca2+. However, upon evaluation with a newly devised, ionomycin-permeabilized cell-attached (Iono-C/A) recording technique, submicromolar concentrations of Ca2+ (apparent Kd = ∼500 nM) were enough to activate this channel. Similar submicromolar Ca2+ dependency was also observed with sharp electrode whole-cell recording and in experiments coexpressing TRPM4 and L-type voltage-dependent Ca2+ channels. Numerical simulations using a number of action potential (AP) models (HL-1, Nygren, Luo-Rudy) incorporating the Ca2+- and voltage-dependent gating parameters of TRPM4, as assessed by Iono-C/A recording, indicated that a few-fold increase in TRPM4 activity is sufficient to delay late AP repolarization and further increases (≥ six-fold) evoke early afterdepolarization. These model predictions are consistent with electrophysiological data from angiotensin II-treated HL-1 cells in which TRPM4 expression and activity were enhanced.

Conclusions

These results collectively indicate that the TRPM4 channel is activated by a physiological range of Ca2+ concentrations and its excessive activity can cause arrhythmic changes. Moreover, these results demonstrate potential utility of the first AP models incorporating TRPM4 gating for in silico assessment of arrhythmogenicity in remodelling cardiac tissue.

Cell-specific mechanisms of TMEM16A Ca2+-activated chloride channel in cancer

TMEM16A (known as anoctamin 1) Ca2+-activated chloride channel is overexpressed in many tumors. TMEM16A overexpression can be caused by gene amplification in many tumors harboring 11q13 amplification. TMEM16A expression is also controlled in many cancer cells via transcriptional regulation, epigenetic regulation and microRNAs. In addition, TMEM16A activates different signaling pathways in different cancers, e.g. the EGFR and CAMKII signaling in breast cancer, the p38 and ERK1/2 signaling in hepatoma, the Ras-Raf-MEK-ERK1/2 signaling in head and neck squamous cell carcinoma and bladder cancer, and the NFκB signaling in glioma. Furthermore, TMEM16A overexpression has been reported to promote, inhibit, or produce no effects on cell proliferation and migration in different cancer cells. Since TMEM16A exerts different roles in different cancer cells via activation of distinct signaling pathways, we try to develop the idea that TMEM16A regulates cancer cell proliferation and migration in a cell-dependent mechanism. The cell-specific role of TMEM16A may depend on the cellular environment that is predetermined by TMEM16A overexpression mechanisms specific for a particular cancer type. TMEM16A may exert its cell-specific role via its associated protein networks, phosphorylation by different kinases, and involvement of different signaling pathways. In addition, we discuss the role of TMEM16A channel activity in cancer, and its clinical use as a prognostic and predictive marker in different cancers. This review highlights the cell-type specific mechanisms of TMEM16A in cancer, and envisions the promising use of TMEM16A inhibitors as a potential treatment for TMEM16A-overexpressing cancers.

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.

Negative News: Cl− and HCO3− in the Vascular Wall

Cl− and HCO3− are the most prevalent membrane-permeable anions in the intra- and extracellular spaces of the vascular wall. Outwardly directed electrochemical gradients for Cl− and HCO3− permit anion channel opening to depolarize vascular smooth muscle and endothelial cells. Transporters and channels for Cl− and HCO3− also modify vascular contractility and structure independently of membrane potential. Transport of HCO3− regulates intracellular pH and thereby modifies the activity of enzymes, ion channels, and receptors. There is also evidence that Cl− and HCO3− transport proteins affect gene expression and protein trafficking. Considering the extensive implications of Cl− and HCO3− in the vascular wall, it is critical to understand how these ions are transported under physiological conditions and how disturbances in their transport can contribute to disease development. Recently, sensing mechanisms for Cl− and HCO3− have been identified in the vascular wall where they modify ion transport and vasomotor function, for instance, during metabolic disturbances. This review discusses current evidence that transport (e.g., via NKCC1, NBCn1, Ca2+-activated Cl− channels, volume-regulated anion channels, and CFTR) and sensing (e.g., via WNK and RPTPγ) of Cl− and HCO3− influence cardiovascular health and disease.

Eact, a small molecule activator of TMEM16A, activates TRPV1 and elicits pain- and itch-related behaviours

BACKGROUND AND PURPOSE

TMEM16A, also known as anoctamin 1 channel, is a member of the Ca(2+)-activated chloride channels family and serves as a heat sensor in the primary nociceptors. Eact is a recently discovered small molecule activator of the TMEM16A channel. Here, we asked if Eact produces pain- and itch-related responses in vivo and investigated the cellular and molecular basis of Eact-elicited responses in dorsal root ganglia (DRG) neurons.

EXPERIMENTAL APPROACH

We employed behavioural testing combined with pharmacological inhibition and genetic ablation approaches to identify transient receptor potential vanilloid 1 (TRPV1) as the prominent mediator for Eact-evoked itch- or pain-related responses. We investigated the effects of Eact on TRPV1 and TMEM16A channels expressed in HEK293T cells and in DRG neurons isolated from wild type and Trpv1(-/-) mice using Ca(2+) imaging and patch-clamp recordings. We also used site-directed mutagenesis to determine the molecular basis of Eact activation of TRPV1.

KEY RESULTS

Administration of Eact elicited both itch- and pain-related behaviours. Unexpectedly, the Eact-elicited behavioural responses were dependent on the function of TRPV1, as shown by pharmacological inhibition and genetic ablation studies. Eact activated membrane currents and increased intracellular free Ca(2+) in both TRPV1-expressing HEK293T cells and isolated DRG neurons in a TRPV1-dependent manner. Eact activation of the TRPV1 channel was severely attenuated by mutations disrupting the capsaicin-binding sites.

CONCLUSIONS AND IMPLICATIONS

Our results suggest that Eact activates primary sensory nociceptors and produces both pain and itch responses mainly through direct activation of TRPV1 channels.

Novel regulatory mechanism in human urinary bladder: central role of transient receptor potential melastatin 4 channels in detrusor smooth muscle function

Transient receptor potential melastatin 4 (TRPM4) channels are Ca(2+)-activated nonselective cation channels that have been recently identified as regulators of detrusor smooth muscle (DSM) function in rodents. However, their expression and function in human DSM remain unexplored. We provide insights into the functional role of TRPM4 channels in human DSM under physiological conditions. We used a multidisciplinary experimental approach, including RT-PCR, Western blotting, immunohistochemistry and immunocytochemistry, patch-clamp electrophysiology, and functional studies of DSM contractility. DSM samples were obtained from patients without preoperative overactive bladder symptoms. RT-PCR detected mRNA transcripts for TRPM4 channels in human DSM whole tissue and freshly isolated single cells. Western blotting and immunohistochemistry with confocal microscopy revealed TRPM4 protein expression in human DSM. Immunocytochemistry further detected TRPM4 protein expression in DSM single cells. Patch-clamp experiments showed that 9-phenanthrol, a selective TRPM4 channel inhibitor, significantly decreased the transient inward cation currents and voltage step-induced whole cell currents in freshly isolated human DSM cells. In current-clamp mode, 9-phenanthrol hyperpolarized the human DSM cell membrane potential. Furthermore, 9-phenanthrol attenuated the spontaneous phasic, carbachol-induced and nerve-evoked contractions in human DSM isolated strips. Significant species-related differences in TRPM4 channel activity between human, rat, and guinea pig DSM were revealed, suggesting a more prominent physiological role for the TRPM4 channel in the regulation of DSM function in humans than in rodents. In conclusion, TRPM4 channels regulate human DSM excitability and contractility and are critical determinants of human urinary bladder function. Thus, TRPM4 channels could represent promising novel targets for the pharmacological or genetic control of overactive bladder.

A Pore Idea: the ion conduction pathway of TMEM16/ANO proteins is composed partly of lipid

Since their first descriptions, ion channels have been conceived as proteinaceous conduits that facilitate the passage of ionic cargo between segregated environments. This concept is reinforced by crystallographic structures of cation channels depicting ion conductance pathways completely lined by protein. Although lipids are sometimes present in fenestrations near the pore or may be involved in channel gating, there is little or no evidence that lipids inhabit the ion conduction pathway. Indeed, the presence of lipid acyl chains in the conductance pathway would curse the design of the channel's aqueous pore. Here, we make a speculative proposal that anion channels in the TMEM16/ANO superfamily have ion conductance pathways composed partly of lipids. Our reasoning is based on the idea that TMEM16 ion channels evolved from a kind of lipid transporter that scrambles lipids between leaflets of the membrane bilayer and the modeled structural similarity between TMEM16 lipid scramblases and TMEM16 anion channels. This novel view of the TMEM16 pore offers explanation for the biophysical and pharmacological oddness of TMEM16A. We build upon the recent X-ray structure of nhTMEM16 and develop models of both TMEM16 ion channels and lipid scramblases to bolster our proposal. It is our hope that this model of the TMEM16 pore will foster innovative investigation into TMEM16 function.

TRPM4 non-selective cation channels influence action potentials in rabbit Purkinje fibres

KEY POINTS

The transient receptor potential melastatin 4 (TRPM4) inhibitor 9-phenanthrol reduces action potential duration in rabbit Purkinje fibres but not in ventricle. TRPM4-like single channel activity is observed in isolated rabbit Purkinje cells but not in ventricular cells. The TRPM4-like current develops during the notch and early repolarization phases of the action potential in Purkinje cells.

ABSTRACT

Transient receptor potential melastatin 4 (TRPM4) Ca(2+)-activated non-selective cation channel activity has been recorded in cardiomyocytes and sinus node cells from mammals. In addition, TRPM4 gene mutations are associated with human diseases of cardiac conduction, suggesting that TRPM4 plays a role in this aspect of cardiac function. Here we evaluate the TRPM4 contribution to cardiac electrophysiology of Purkinje fibres. Ventricular strips with Purkinje fibres were isolated from rabbit hearts. Intracellular microelectrodes recorded Purkinje fibre activity and the TRPM4 inhibitor 9-phenanthrol was applied to unmask potential TRPM4 contributions to the action potential. 9-Phenanthrol reduced action potential duration measured at the point of 50 and 90% repolarization with an EC50 of 32.8 and 36.1×10(-6) mol l(-1), respectively, but did not modulate ventricular action potentials. Inside-out patch-clamp recordings were used to monitor TRPM4 activity in isolated Purkinje cells. TRPM4-like single channel activity (conductance = 23.8 pS; equal permeability for Na(+) and K(+); sensitivity to voltage, Ca(2+) and 9-phenanthrol) was observed in 43% of patches from Purkinje cells but not from ventricular cells (0/16). Action potential clamp experiments performed in the whole-cell configuration revealed a transient inward 9-phenanthrol-sensitive current (peak density = -0.65 ± 0.15 pA pF(-1); n = 5) during the plateau phases of the Purkinje fibre action potential. These results show that TRPM4 influences action potential characteristics in rabbit Purkinje fibres and thus could modulate cardiac conduction and be involved in triggering arrhythmias.

A TRPM4-dependent current in murine renal primary cilia

Defects in primary cilia lead to a variety of human diseases. One of these, polycystic kidney disease, can be caused by defects in a Ca²⁺-gated ion channel (TRPP2) found on the cilium. Other ciliary functions also contribute to cystogenesis, and defects in apical Ca²⁺ homeostasis have been implicated. By recording directly from the native cilia of mIMCD-3 cells, a murine cell line of renal epithelial origin, we have identified a second Ca²⁺-gated channel in the ciliary membrane: the transient receptor potential cation channel, subfamily M, member 4 (TRPM4). In excised primary cilia, TRPM4 was found to have a low sensitivity to Ca²⁺, with an EC₅₀ of 646 μM at +100 mV. It was inhibited by MgATP and by 9-phenanthrol. The channel was not permeable to Ca²⁺ or Cl⁻ and had a permeability ratio PK/PNa of 1.42. Reducing the expression of Trpm4 mRNA with short hairpin (sh) RNA reduced the TRPM4 current by 87% and shortened primary cilia by 43%. When phospholipase C was inhibited, the sensitivity to cytoplasmic Ca²⁺ greatly increased (EC₅₀ = 26 μM at +100 mV), which is consistent with previous reports that phosphatidylinositol 4,5-bisphosphate (PIP2) modulates the channel. MgATP did not restore the channel to a preinactivation state, suggesting that the enzyme or substrate necessary for making PIP2 is not abundant in primary cilia of mIMCD-3 cells. The function of TRPM4 in renal primary cilia is not yet known, but it is likely to influence the apical Ca²⁺ dynamics of the cell, perhaps in tandem with TRPP2.

TRPM4 in cardiac electrical activity

TRPM4 forms a non-selective cation channel activated by internal Ca(2+). Its functional expression was demonstrated in cardiomyocytes of several mammalian species including humans, but the channel is also present in many other tissues. The recent characterization of the TRPM4 inhibitor 9-phenanthrol, and the availability of transgenic mice have helped to clarify the role of TRPM4 in cardiac electrical activity, including diastolic depolarization from the sino-atrial node cells in mouse, rat, and rabbit, as well as action potential duration in mouse cardiomyocytes. In rat and mouse, pharmacological inhibition of TRPM4 prevents cardiac ischaemia-reperfusion injuries and decreases the occurrence of arrhythmias. Several studies have identified TRPM4 mutations in patients with inherited cardiac diseases including conduction blocks and Brugada syndrome. This review identifies TRPM4 as a significant actor in cardiac electrophysiology.

The Ca(2+)-activated cation channel TRPM4 is a negative regulator of angiotensin II-induced cardiac hypertrophy

Cardiac muscle adapts to hemodynamic stress by altering myocyte size and function, resulting in cardiac hypertrophy. Alteration in myocyte calcium homeostasis is known to be an initial signal in cardiac hypertrophy signaling. Transient receptor potential melastatin 4 protein (TRPM4) is a calcium-activated non-selective cation channel, which plays a role in regulating calcium influx and calcium-dependent cell functions in many cell types including cardiomyocytes. Selective deletion of TRPM4 from the heart muscle in mice resulted in an increased hypertrophic growth after chronic angiotensin (AngII) treatment, compared to WT mice. The enhanced hypertrophic response was also traceable by the increased expression of hypertrophy-related genes like Rcan1, ANP, and α-Actin. Intracellular calcium measurements on isolated ventricular myocytes showed significantly increased store-operated calcium entry upon AngII treatment in myocytes lacking the TRPM4 channel. Elevated intracellular calcium is a key factor in the development of pathological cardiac hypertrophy, leading to the activation of intracellular signaling pathways. In agreement with this, we observed significantly higher Rcan1 mRNA level, calcineurin enzyme activity and protein level in lysates from TRPM4-deficient mice heart compared to WT after AngII treatment. Collectively, these observations are consistent with a model in which TRPM4 is a regulator of calcium homeostasis in cardiomyocytes after AngII stimulation. TRPM4 contributes to the regulation of driving force for store-operated calcium entry and thereby the activation of the calcineurin–NFAT pathway and the development of pathological hypertrophy.