Voltage dependence of the Ca2+-activated cation channel TRPM4

Recent advances in therapeutic strategies that focus on the regulation of ion channel expression

A number of different ion channel types are involved in cell signaling networks, and homeostatic regulatory mechanisms contribute to the control of ion channel expression. Profiling of global gene expression using microarray technology has recently provided novel insights into the molecular mechanisms underlying the homeostatic and pathological control of ion channel expression. It has demonstrated that the dysregulation of ion channel expression is associated with the pathogenesis of neural, cardiovascular, and immune diseases as well as cancers. In addition to the transcriptional, translational, and post-translational regulation of ion channels, potentially important evidence on the mechanisms controlling ion channel expression has recently been accumulated. The regulation of alternative pre-mRNA splicing is therefore a novel therapeutic strategy for the treatment of dominant-negative splicing disorders. Epigenetic modification plays a key role in various pathological conditions through the regulation of pluripotency genes. Inhibitors of pre-mRNA splicing and histone deacetyalase/methyltransferase have potential as potent therapeutic drugs for cancers and autoimmune and inflammatory diseases. Moreover, membrane-anchoring proteins, lysosomal and proteasomal degradation-related molecules, auxiliary subunits, and pharmacological agents alter the protein folding, membrane trafficking, and post-translational modifications of ion channels, and are linked to expression-defect channelopathies. In this review, we focused on recent insights into the transcriptional, spliceosomal, epigenetic, and proteasomal regulation of ion channel expression: Ca(2+) channels (TRPC/TRPV/TRPM/TRPA/Orai), K(+) channels (voltage-gated, KV/Ca(2+)-activated, KCa/two-pore domain, K2P/inward-rectifier, Kir), and Ca(2+)-activated Cl(-) channels (TMEM16A/TMEM16B). Furthermore, this review highlights expression of these ion channels in expression-defect channelopathies.

Shear stress activates monovalent cation channel transient receptor potential melastatin subfamily 4 in rat atrial myocytes via type 2 inositol 1,4,5-trisphosphate receptors and Ca(2+) release

KEY POINTS

During each contraction and haemodynamic disturbance, cardiac myocytes are subjected to fluid shear stress as a result of blood flow and the relative movement of sheets of myocytes. The present study aimed to characterize the shear stress-sensitive membrane current in atrial myocytes using the whole-cell patch clamp technique, combined with pressurized fluid flow, as well as pharmacological and genetic interventions of specific proteins. The data obtained suggest that shear stress indirectly activates the monovalent cation current carried by transient receptor potential melastatin subfamily 4 channels via type 2 inositol 1,4,5-trisphosphate receptor-mediated Ca(2+) release in subsarcolemmal domains of atrial myocytes. Ca(2+) -mediated interactions between these two proteins under shear stress may be an important mechanism by which atrial cells measure mechanical stress and translate it to alter their excitability.

ABSTRACT

Atrial myocytes are subjected to shear stress during the cardiac cycle under physiological or pathological conditions. The ionic currents regulated by shear stress remain poorly understood. We report the characteristics, molecular identity and activation mechanism of the shear stress-sensitive current (Ishear ) in rat atrial myocytes. A shear stress of ∼16 dyn cm(-2) was applied to single myocytes using a pressurized microflow system, and the current was measured by whole-cell patch clamp. In symmetrical CsCl solutions with minimal concentrations of internal EGTA, Ishear showed an outwardly rectifying current-voltage relationship (reversal at -2 mV). The current was conducted primarily (∼80%) by monovalent cations but not Ca(2+) . It was suppressed by intracellular Ca(2+) buffering at a fixed physiological level, inhibitors of transient receptor potential melastatin subfamily 4 (TRPM4), intracellular introduction of TRPM4 antibodies or knockdown of TRPM4 expression, suggesting that TRPM4 carries most of this current. A notable reduction in Ishear occurred upon inhibition of Ca(2+) release through the ryanodine receptors or inositol 1,4,5-trisphosphate receptors (IP3 R) and upon depletion of sarcoplasmic reticulum Ca(2+) . In type 2 IP3 R (IP3 R2) knockout atrial myocytes, Ishear was 10-20% of that in wild-type myocytes. Immunocytochemistry and proximity ligation assays revealed that TRPM4 and IP3 R2 were expressed at peripheral sites with co-localization, although they are not localized within 40 nm. Peripheral localization of TRPM4 was intact in IP3 R2 knockout cells. The data obtained in the present study suggest that shear stress activates TRPM4 current by triggering Ca(2+) release from the IP3 R2 in the peripheral domains of atrial myocytes.

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.

TRPM4-dependent post-synaptic depolarization is essential for the induction of NMDA receptor-dependent LTP in CA1 hippocampal neurons

TRPM4 is a calcium-activated but calcium-impermeable non-selective cation (CAN) channel. Previous studies have shown that TRPM4 is an important regulator of Ca(2+)-dependent changes in membrane potential in excitable and non-excitable cell types. However, its physiological significance in neurons of the central nervous system remained unclear. Here, we report that TRPM4 proteins form a CAN channel in CA1 neurons of the hippocampus and we show that TRPM4 is an essential co-activator of N-methyl-D-aspartate (NMDA) receptors (NMDAR) during the induction of long-term potentiation (LTP). Disrupting the Trpm4 gene in mice specifically eliminates NMDAR-dependent LTP, while basal synaptic transmission, short-term plasticity, and NMDAR-dependent long-term depression are unchanged. The induction of LTP in Trpm4 (-/-) neurons was rescued by facilitating NMDA receptor activation or post-synaptic membrane depolarization. Accordingly, we obtained normal LTP in Trpm4 (-/-) neurons in a pairing protocol, where post-synaptic depolarization was applied in parallel to pre-synaptic stimulation. Taken together, our data are consistent with a novel model of LTP induction in CA1 hippocampal neurons, in which TRPM4 is an essential player in a feed-forward loop that generates the post-synaptic membrane depolarization which is necessary to fully activate NMDA receptors during the induction of LTP but which is dispensable for the induction of long-term depression (LTD). These results have important implications for the understanding of the induction process of LTP and the development of nootropic medication.

Expression and functionality of transient receptor potential melastatin 4 (TRPM4)-like channels during development of the zebrafish

Calcium signaling, from localized spikes to coordinated waves, are linked to cleavage, patterning, differentiation, and growth during embryonic development. The basis for control of these Ca(2+) signals is poorly defined. In this study, the expression and functionality of the transient receptor potential melastatin 4 protein (TRPM4), an ion channel that controls Ca(2+) entry into cells, was examined in the zebrafish embryo and adult. Originating with the human TRPM4 gene, Ensembl ortholog, NCBI BLAST, and Homologene searches identified a zebrafish TRPM4 "like" gene encoding a predicted protein of 1199 amino acids and sharing a 42-43% sequence identity with the mouse, rat, and human. Custom-designed zebrafish primers identified TRPM4 transcripts throughout the 0-123h period of embryonic development with greatest and lowest relative expression at 12 and 123h post-fertilization, respectively. Perforated patch clamp recordings in 27h embryonic cells revealed Ca(2+)-activated currents with the characteristics of those described for mammalian TRPM4. Similarly, TRPM4-like expression and functionality was observed in brain and liver cells from adult fish. These findings suggest that a TRPM4-like channel is available for Ca(2+) regulation during early development of the zebrafish.

GLP-1 stimulates insulin secretion by PKC-dependent TRPM4 and TRPM5 activation

Strategies aimed at mimicking or enhancing the action of the incretin hormone glucagon-like peptide 1 (GLP-1) therapeutically improve glucose-stimulated insulin secretion (GSIS); however, it is not clear whether GLP-1 directly drives insulin secretion in pancreatic islets. Here, we examined the mechanisms by which GLP-1 stimulates insulin secretion in mouse and human islets. We found that GLP-1 enhances GSIS at a half-maximal effective concentration of 0.4 pM. Moreover, we determined that GLP-1 activates PLC, which increases submembrane diacylglycerol and thereby activates PKC, resulting in membrane depolarization and increased action potential firing and subsequent stimulation of insulin secretion. The depolarizing effect of GLP-1 on electrical activity was mimicked by the PKC activator PMA, occurred without activation of PKA, and persisted in the presence of PKA inhibitors, the KATP channel blocker tolbutamide, and the L-type Ca(2+) channel blocker isradipine; however, depolarization was abolished by lowering extracellular Na(+). The PKC-dependent effect of GLP-1 on membrane potential and electrical activity was mediated by activation of Na(+)-permeable TRPM4 and TRPM5 channels by mobilization of intracellular Ca(2+) from thapsigargin-sensitive Ca(2+) stores. Concordantly, GLP-1 effects were negligible in Trpm4 or Trpm5 KO islets. These data provide important insight into the therapeutic action of GLP-1 and suggest that circulating levels of this hormone directly stimulate insulin secretion by β cells.

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.

PIP2 and PIP3 interact with N-terminus region of TRPM4 channel

The transient receptor potential melastatin 4 (TRPM4) is a calcium-activated non-selective ion channel broadly expressed in a variety of tissues. Receptor has been identified as a crucial modulator of numerous calcium dependent mechanisms in the cell such as immune response, cardiac conduction, neurotransmission and insulin secretion. It is known that phosphoinositide lipids (PIPs) play a unique role in the regulation of TRP channel function. However the molecular mechanism of this process is still unknown. We characterized the binding site of PIP2 and its structural analogue PIP3 in the E733-W772 proximal region of the TRPM4 N-terminus via biophysical and molecular modeling methods. The specific positions R755 and R767 in this domain were identified as being important for interactions with PIP2/PIP3 ligands. Their mutations caused a partial loss of PIP2/PIP3 binding specificity. The interaction of PIP3 with TRPM4 channels has never been described before. These findings provide new insight into the ligand binding domains of the TRPM4 channel.

Transient receptor potential channels in the vasculature

The mammalian genome encodes 28 distinct members of the transient receptor potential (TRP) superfamily of cation channels, which exhibit varying degrees of selectivity for different ionic species. Multiple TRP channels are present in all cells and are involved in diverse aspects of cellular function, including sensory perception and signal transduction. Notably, TRP channels are involved in regulating vascular function and pathophysiology, the focus of this review. TRP channels in vascular smooth muscle cells participate in regulating contractility and proliferation, whereas endothelial TRP channel activity is an important contributor to endothelium-dependent vasodilation, vascular wall permeability, and angiogenesis. TRP channels are also present in perivascular sensory neurons and astrocytic endfeet proximal to cerebral arterioles, where they participate in the regulation of vascular tone. Almost all of these functions are mediated by changes in global intracellular Ca(2+) levels or subcellular Ca(2+) signaling events. In addition to directly mediating Ca(2+) entry, TRP channels influence intracellular Ca(2+) dynamics through membrane depolarization associated with the influx of cations or through receptor- or store-operated mechanisms. Dysregulation of TRP channels is associated with vascular-related pathologies, including hypertension, neointimal injury, ischemia-reperfusion injury, pulmonary edema, and neurogenic inflammation. In this review, we briefly consider general aspects of TRP channel biology and provide an in-depth discussion of the functions of TRP channels in vascular smooth muscle cells, endothelial cells, and perivascular cells under normal and pathophysiological conditions.

Vascular TRP channels: performing under pressure and going with the flow

Endothelial cells and smooth muscle cells of resistance arteries mediate opposing responses to mechanical forces acting on the vasculature, promoting dilation in response to flow and constriction in response to pressure, respectively. In this review, we explore the role of TRP channels, particularly endothelial TRPV4 and smooth muscle TRPC6 and TRPM4 channels, in vascular mechanosensing circuits, placing their putative mechanosensitivity in context with other proposed upstream and downstream signaling pathways.

Placing ion channels into a signaling network of T cells: from maturing thymocytes to healthy T lymphocytes or leukemic T lymphoblasts

T leukemogenesis is a multistep process, where the genetic errors during T cell maturation cause the healthy progenitor to convert into the leukemic precursor that lost its ability to differentiate but possesses high potential for proliferation, self-renewal, and migration. A new misdirecting "leukemogenic" signaling network appears, composed by three types of participants which are encoded by (1) genes implicated in determined stages of T cell development but deregulated by translocations or mutations, (2) genes which normally do not participate in T cell development but are upregulated, and (3) nondifferentially expressed genes which become highly interconnected with genes expressed differentially. It appears that each of three groups may contain genes coding ion channels. In T cells, ion channels are implicated in regulation of cell cycle progression, differentiation, activation, migration, and cell death. In the present review we are going to reveal a relationship between different genetic defects, which drive the T cell neoplasias, with calcium signaling and ion channels. We suggest that changes in regulation of various ion channels in different types of the T leukemias may provide the intracellular ion microenvironment favorable to maintain self-renewal capacity, arrest differentiation, induce proliferation, and enhance motility.

Trpm4 gene invalidation leads to cardiac hypertrophy and electrophysiological alterations

RATIONALE

TRPM4 is a non-selective Ca2+-activated cation channel expressed in the heart, particularly in the atria or conduction tissue. Mutations in the Trpm4 gene were recently associated with several human conduction disorders such as Brugada syndrome. TRPM4 channel has also been implicated at the ventricular level, in inotropism or in arrhythmia genesis due to stresses such as ß-adrenergic stimulation, ischemia-reperfusion, and hypoxia re-oxygenation. However, the physiological role of the TRPM4 channel in the healthy heart remains unclear.

OBJECTIVES

We aimed to investigate the role of the TRPM4 channel on whole cardiac function with a Trpm4 gene knock-out mouse (Trpm4-/-) model.

METHODS AND RESULTS

Morpho-functional analysis revealed left ventricular (LV) eccentric hypertrophy in Trpm4-/- mice, with an increase in both wall thickness and chamber size in the adult mouse (aged 32 weeks) when compared to Trpm4+/+ littermate controls. Immunofluorescence on frozen heart cryosections and qPCR analysis showed no fibrosis or cellular hypertrophy. Instead, cardiomyocytes in Trpm4-/- mice were smaller than Trpm4+/+with a higher density. Immunofluorescent labeling for phospho-histone H3, a mitosis marker, showed that the number of mitotic myocytes was increased 3-fold in the Trpm4-/-neonatal stage, suggesting hyperplasia. Adult Trpm4-/- mice presented multilevel conduction blocks, as attested by PR and QRS lengthening in surface ECGs and confirmed by intracardiac exploration. Trpm4-/-mice also exhibited Luciani-Wenckebach atrioventricular blocks, which were reduced following atropine infusion, suggesting paroxysmal parasympathetic overdrive. In addition, Trpm4-/- mice exhibited shorter action potentials in atrial cells. This shortening was unrelated to modifications of the voltage-gated Ca2+ or K+ currents involved in the repolarizing phase.

CONCLUSIONS

TRPM4 has pleiotropic roles in the heart, including the regulation of conduction and cellular electrical activity which impact heart development.

Calcium channels in Fc receptor signaling

The calcium ion (Ca(2+)) is the main common second messenger involved in signaling transduction subsequent to immunoreceptor activation. Its rapid intracellular elevation induces multiple cellular responses, such as secretion, proliferation, mobility, and gene transcription. Intracellular levels of Ca(2+) need to reach a specific threshold to efficiently transduce the signal to activate transcription factors through the recruitment of Ca(2+)-binding molecules. However, since Ca(2+) cannot be metabolized, its intracellular concentration is tightly regulated to avoid the induction of programmed cell death. This highly controlled regulation of Ca(2+) homeostasis has recently been clarified by the uncovering of new ion channels. The regulation of these channels allows the role of Ca(2+) in Fc receptor transduction pathways to be more precisely defined.

Negatively charged amino acids near and in transient receptor potential (TRP) domain of TRPM4 channel are one determinant of its Ca2+ sensitivity

Transient receptor potential (TRP) channel melastatin subfamily member 4 (TRPM4) is a broadly expressed nonselective monovalent cation channel. TRPM4 is activated by membrane depolarization and intracellular Ca(2+), which is essential for the activation. The Ca(2+) sensitivity is known to be regulated by calmodulin and membrane phosphoinositides, such as phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2). Although these regulators must play important roles in controlling TRPM4 activity, mutation analyses of the calmodulin-binding sites have suggested that Ca(2+) binds to TRPM4 directly. However, the intrinsic binding sites in TRPM4 remain to be elucidated. Here, by using patch clamp and molecular biological techniques, we show that there are at least two functionally different divalent cation-binding sites, and the negatively charged amino acids near and in the TRP domain in the C-terminal tail of TRPM4 (Asp-1049 and Glu-1062 of rat TRPM4) are required for maintaining the normal Ca(2+) sensitivity of one of the binding sites. Applications of Co(2+), Mn(2+), or Ni(2+) to the cytosolic side potentiated TRPM4 currents, increased the Ca(2+) sensitivity, but were unable to evoke TRPM4 currents without Ca(2+). Mutations of the acidic amino acids near and in the TRP domain, which are conserved in TRPM2, TRPM5, and TRPM8, deteriorated the Ca(2+) sensitivity in the presence of Co(2+) or PI(4,5)P2 but hardly affected the sensitivity to Co(2+) and PI(4,5)P2. These results suggest a novel role of the TRP domain in TRPM4 as a site responsible for maintaining the normal Ca(2+) sensitivity. These findings provide more insights into the molecular mechanisms of the regulation of TRPM4 by Ca(2+).

Differential contribution of TRPM4 and TRPM5 nonselective cation channels to the slow afterdepolarization in mouse prefrontal cortex neurons

In certain neurons from different brain regions, a brief burst of action potentials can activate a slow afterdepolarization (sADP) in the presence of muscarinic acetylcholine receptor agonists. The sADP, if suprathreshold, can contribute to persistent non-accommodating firing in some of these neurons. Previous studies have characterized a Ca²⁺-activated non-selective cation (CAN) current (I_CAN) that is thought to underlie the sADP. I_CAN depends on muscarinic receptor stimulation and exhibits a dependence on neuronal activity, membrane depolarization and Ca²⁺-influx similar to that observed for the sADP. Despite the widespread occurrence of sADPs in neurons throughout the brain, the molecular identity of the ion channels underlying these events, as well as I_CAN, remains uncertain. Here we used a combination of genetic, pharmacological and electrophysiological approaches to characterize the molecular mechanisms underlying the muscarinic receptor-dependent sADP in layer 5 pyramidal neurons of mouse prefrontal cortex. First, we confirmed that in the presence of the cholinergic agonist carbachol a brief burst of action potentials triggers a prominent sADP in these neurons. Second, we confirmed that this sADP requires activation of a PLC signaling cascade and intracellular calcium signaling. Third, we obtained direct evidence that the transient receptor potential (TRP) melastatin 5 channel (TRPM5), which is thought to function as a CAN channel in non-neural cells, contributes importantly to the sADP in the layer 5 neurons. In contrast, the closely related TRPM4 channel may play only a minor role in the sADP.

Permeation, regulation and control of expression of TRP channels by trace metal ions

Transient receptor potential (TRP) channels form a diverse family of cation channels comprising 28 members in mammals. Although some TRP proteins can only be found on intracellular membranes, most of the TRP protein isoforms reach the plasma membrane where they form ion channels and control a wide number of biological processes. There, their involvement in the transport of cations such as calcium and sodium has been well documented. However, a growing number of studies have started to expand our understanding of these proteins by showing that they also transport other biologically relevant metal ions like zinc, magnesium, manganese and cobalt. In addition to this newly recognized property, the activity and expression of TRP channels can be regulated by metal ions like magnesium, gadolinium, lanthanum or cisplatin. The aim of this review is to highlight the complex relationship between metal ions and TRP channels.

Adenylyl cyclase-mediated effects contribute to increased Isoprenaline-induced cardiac contractility in TRPM4-deficient mice

TRPM4 and TRPM5 proteins belong to the Transient Receptor Potential (TRP) ion channel family and form Ca(2+)-activated nonselective cation channels. Recently we showed a significant increase of Isoprenaline-induced inotropy in TRPM4-deficient (Trpm4(-/-)) mice. This is caused by increased Ca(2+) entry via L-type calcium channels due to faster action potential repolarization in Trpm4(-/-) ventricular myocytes [Mathar et al., 2013]. Here, we investigated the contribution of various steps of the β-adrenergic signalling cascade to the augmented positive inotropic response in the absence of TRPM4, and whether the closely related TRPM5 additively contributes to this process using TRPM4/TRPM5-double deficient (Trpm4/Trpm5((-/-)2)) mice. We performed contractility measurements on isolated papillary muscles from wild type, Trpm4(-/-) and Trpm4/Trpm5((-/-)2) mice. As shown in Trpm4(-/-) mice, Isoprenaline-induced inotropy in Trpm4/Trpm5((-/-)2) papillary muscles was significantly increased compared to wild type, whereas basal, frequency- and Ca(2+)-dependent contractility was unaltered. Equivalent to Isoprenaline, activation of adenylyl cyclase using Forskolin led to a significantly increased twitch force in Trpm4(-/-) heart preparations whereas the Isoprenaline-mediated increase in cAMP level was comparable to wild type mice. Notably, the positive inotropic response evoked by phosphodiesterase inhibition with 3-isobutyl-1-methylxanthine (IBMX) was unchanged between both genotypes. Furthermore, experiments performed with increasing concentrations of IBMX after prestimulation with Forskolin and vice versa did not provide evidence that the increased β-adrenergic positive inotropic response in TRPM4-deficient papillary muscles is due to differences in accumulation of cAMP. Compared to inhibition of phosphodiesterase, the rise of intracellular cAMP by activating adenylyl cyclase is accompanied by ATP breakdown. To test the relevance of TRPM4 during forced ATP consumption we measured contractility under ischemic conditions. Here, Trpm4(-/-) papillary muscles showed improved contractile function in comparison to wild type. Our results are consistent with the hypothesis that TRPM4 has a limiting effect on cardiac contractility specifically in ATP depleting conditions. The increased positive inotropic response in Trpm4(-/-) papillary muscles evoked by stimulation of adenylyl cyclase activity is not observed without active enhancement of ATP hydrolysis. Furthermore, the contractility of Trpm4(-/-) papillary muscles was also increased during ischemic simulation. These data underscore the potential of TRPM4 inactivation as an approach to increase inotropy in specific conditions associated with increased catecholamine levels, such as heart failure and ischemia.

A PLCγ1-dependent, force-sensitive signaling network in the myogenic constriction of cerebral arteries

Maintaining constant blood flow in the face of fluctuations in blood pressure is a critical autoregulatory feature of cerebral arteries. An increase in pressure within the artery lumen causes the vessel to constrict through depolarization and contraction of the encircling smooth muscle cells. This pressure-sensing mechanism involves activation of two types of transient receptor potential (TRP) channels: TRPC6 and TRPM4. We provide evidence that the activation of the γ1 isoform of phospholipase C (PLCγ1) is critical for pressure sensing in cerebral arteries. Inositol 1,4,5-trisphosphate (IP3), generated by PLCγ1 in response to pressure, sensitized IP3 receptors (IP3Rs) to Ca(2+) influx mediated by the mechanosensitive TRPC6 channel, synergistically increasing IP3R-mediated Ca(2+) release to activate TRPM4 currents, leading to smooth muscle depolarization and constriction of isolated cerebral arteries. Proximity ligation assays demonstrated colocalization of PLCγ1 and TRPC6 with TRPM4, suggesting the presence of a force-sensitive, local signaling network comprising PLCγ1, TRPC6, TRPM4, and IP3Rs. Src tyrosine kinase activity was necessary for stretch-induced TRPM4 activation and myogenic constriction, consistent with the ability of Src to activate PLCγ isoforms. We conclude that contraction of cerebral artery smooth muscle cells requires the integration of pressure-sensing signaling pathways and their convergence on IP3Rs, which mediate localized Ca(2+)-dependent depolarization through the activation of TRPM4.

Endotoxin induces fibrosis in vascular endothelial cells through a mechanism dependent on transient receptor protein melastatin 7 activity

The pathogenesis of systemic inflammatory diseases, including endotoxemia-derived sepsis syndrome, is characterized by endothelial dysfunction. It has been demonstrated that the endotoxin lipopolysaccharide (LPS) induces the conversion of endothelial cells (ECs) into activated fibroblasts through endothelial-to-mesenchymal transition mechanism. Fibrogenesis is highly dependent on intracellular Ca2+ concentration increases through the participation of calcium channels. However, the specific molecular identity of the calcium channel that mediates the Ca2+ influx during endotoxin-induced endothelial fibrosis is still unknown. Transient receptor potential melastatin 7 (TRPM7) is a calcium channel that is expressed in many cell types, including ECs. TRPM7 is involved in a number of crucial processes such as the conversion of fibroblasts into activated fibroblasts, or myofibroblasts, being responsible for the development of several characteristics of them. However, the role of the TRPM7 ion channel in endotoxin-induced endothelial fibrosis is unknown. Thus, our aim was to study whether the TRPM7 calcium channel participates in endotoxin-induced endothelial fibrosis. Using primary cultures of ECs, we demonstrated that TRPM7 is a crucial protein involved in endotoxin-induced endothelial fibrosis. Suppression of TRPM7 expression protected ECs from the fibrogenic process stimulated by endotoxin. Downregulation of TRPM7 prevented the endotoxin-induced endothelial markers decrease and fibrotic genes increase in ECs. In addition, TRPM7 downregulation abolished the endotoxin-induced increase in ECM proteins in ECs. Furthermore, we showed that intracellular Ca2+ levels were greatly increased upon LPS challenge in a mechanism dependent on TRPM7 expression. These results demonstrate that TRPM7 is a key protein involved in the mechanism underlying endotoxin-induced endothelial fibrosis.

Calcium-permeable ion channels in pain signaling

The detection and processing of painful stimuli in afferent sensory neurons is critically dependent on a wide range of different types of voltage- and ligand-gated ion channels, including sodium, calcium, and TRP channels, to name a few. The functions of these channels include the detection of mechanical and chemical insults, the generation of action potentials and regulation of neuronal firing patterns, the initiation of neurotransmitter release at dorsal horn synapses, and the ensuing activation of spinal cord neurons that project to pain centers in the brain. Long-term changes in ion channel expression and function are thought to contribute to chronic pain states. Many of the channels involved in the afferent pain pathway are permeable to calcium ions, suggesting a role in cell signaling beyond the mere generation of electrical activity. In this article, we provide a broad overview of different calcium-permeable ion channels in the afferent pain pathway and their role in pain pathophysiology.

Increases in reactive oxygen species enhance vascular endothelial cell migration through a mechanism dependent on the transient receptor potential melastatin 4 ion channel

A hallmark of severe inflammation is reactive oxygen species (ROS) overproduction induced by increased inflammatory mediators secretion. During systemic inflammation, inflammation mediators circulating in the bloodstream interact with endothelial cells (ECs) raising intracellular oxidative stress at the endothelial monolayer. Oxidative stress mediates several pathological functions, including an exacerbated EC migration. Because cell migration critically depends on calcium channel-mediated Ca(2+) influx, the molecular identification of the calcium channel involved in oxidative stress-modulated EC migration has been the subject of intense investigation. The transient receptor potential melastatin 4 (TRPM4) protein is a ROS-modulated non-selective cationic channel that performs several cell functions, including regulating intracellular Ca(2+) overload and Ca(2+) oscillation. This channel is expressed in multiple tissues, including ECs, and contributes to the migration of certain immune cells. However, whether the TRPM4 ion channel participates in oxidative stress-mediated EC migration is not known. Herein, we investigate whether oxidative stress initiates or enhances EC migration and study the role played by the ROS-modulated TRPM4 ion channel in oxidative stress-mediated EC migration. We demonstrate that oxidative stress enhances, but does not initiate, EC migration in a dose-dependent manner. Notably, we demonstrate that the TRPM4 ion channel is critical in promoting H2O2-enhanced EC migration. These results show that TRPM4 is a novel pharmacological target for the possible treatment of severe inflammation and other oxidative stress-mediated inflammatory diseases.

TRPM5

TRPM5 is a Ca(2+)-activated cation channel that mediates signaling in taste and other chemosensory cells. Within taste cells, TRPM5 is the final element in a signaling cascade that starts with the activation of G protein-coupled receptors by bitter, sweet, or umami taste molecules and that requires the enzyme PLCβ2. PLCβ2 breaks down PIP2 into DAG and IP3, and the ensuing release of Ca(2+) from intracellular stores activates TRPM5. Since its initial discovery in the taste system, TRPM5 has been found to be distributed in sparse chemosensory cells located throughout the digestive track, in the respiratory system, and in the olfactory system. It is also found in pancreatic islets, where it contributes to insulin secretion. This review highlights recent work on the mechanisms of the activation of the TRPM5 channel and its regulation by voltage, phosphoinositides, temperature, and pH. The distribution of the channel in the body and its functional contribution to various sensory and nonsensory processes are discussed.

TRPM4

TRPM4 is a Ca(2+)-activated nonselective cation channel. The channel is activated by an increase of intracellular Ca(2+) and is regulated by several factors including temperature and Pi(4,5)P2. TRPM4 allows Na(+) entry into the cell upon activation, but is completely impermeable to Ca(2+). Unlike TRPM5, its closest relative in the transient receptor potential family, TRPM4 proteins are widely expressed in the body. Currents with properties that are reminiscent of TRPM4 have been described in a variety of tissues since the advent of the patch clamp technology, but their physiological role is only beginning to be clarified with the increasing characterization of knockout mouse models for TRPM4. Furthermore, mutations in the TRPM4 gene have been associated with cardiac conduction disorders in human patients. This review aims to overview the currently available data on the functional properties of TRPM4 and the current understanding of its physiological role in healthy and diseased tissue.

TRPV6 channels

TRPV6 (former synonyms ECAC2, CaT1, CaT-like) displays several specific features which makes it unique among the members of the mammalian Trp gene family (1) TRPV6 (and its closest relative, TRPV5) are the only highly Ca(2+)-selective channels of the entire TRP superfamily (Peng et al. 1999; Wissenbach et al. 2001; Voets et al. 2004). (2) Translation of Trpv6 initiates at a non-AUG codon, at ACG, located upstream of the annotated AUG, which is not used for initiation (Fecher-Trost et al. 2013). The ACG codon is nevertheless decoded by methionine. Not only a very rare event in eukaryotic biology, the full-length TRPV6 protein existing in vivo comprises an amino terminus extended by 40 amino acid residues compared to the annotated truncated TRPV6 protein which has been used in most studies on TRPV6 channel activity so far. (In the following numbering occurs according to this full-length protein, with the numbers of the so far annotated truncated protein in brackets). (3) Only in humans a coupled polymorphism of Trpv6 exists causing three amino acid exchanges and resulting in an ancestral Trpv6 haplotype and a so-called derived Trpv6 haplotype (Wissenbach et al. 2001). The ancestral allele encodes the amino acid residues C197(157), M418(378) and M721(681) and the derived alleles R197(157), V418(378) and T721(681). The ancestral haplotype is found in all species, the derived Trpv6 haplotype has only been identified in humans, and its frequency increases with the distance to the African continent. Apparently the Trpv6 gene has been a strong target for selection in humans, and its derived variant is one of the few examples showing consistently differences to the orthologues genes of other primates (Akey et al. 2004, 2006; Stajich and Hahn 2005; Hughes et al. 2008). (4) The Trpv6 gene expression is significantly upregulated in several human malignancies including the most common cancers, prostate and breast cancer (Wissenbach et al. 2001; Zhuang et al. 2002; Fixemer et al. 2003; Bolanz et al. 2008). (5) Male mice lacking functional TRPV6 channels are hypo-/infertile making TRPV6 one of the very few channels essential for male fertility (Weissgerber et al. 2011, 2012).

Gating of thermally activated channels

A class of ion channels that belongs to the transient receptor potential (TRP) superfamily and is present in specialized neurons that project to the skin has evolved as temperature detectors. These channels are classified into subfamilies, namely canonical (TRPC), melastatin (TRPM), ankyrin (TRPA), and vanilloid (TRPV). Some of these channels are activated by heat (TRPM2/4/5, TRPV1-4), while others by cold (TRPA1, TRPC5, and TRPM8). The general structure of these channels is closely related to that of the voltage-dependent K(+) channels, with their subunits containing six transmembrane segments that form tetramers. Thermal TRP channels are polymodal receptors. That is, they can be activated by temperature, voltage, pH, lipids, and agonists. The high temperature sensitivity in these thermal TRP channels is due to a large enthalpy change (∼100 kcal/mol), which is about five times the enthalpy change in voltage-dependent gating. The characterization of the macroscopic currents and single-channel analysis demonstrated that gating by temperature is complex and best described by branched or allosteric models containing several closed and open states. The identification of molecular determinants of temperature sensitivity in TRPV1, TRPA1, and TRPV3 strongly suggest that thermal sensitivity arises from a specific protein domain.

Regulation of cerebral artery smooth muscle membrane potential by Ca²⁺-activated cation channels

Arterial tone is dependent on the depolarizing and hyperpolarizing currents regulating membrane potential and governing the influx of Ca²⁺ needed for smooth muscle contraction. Several ion channels have been proposed to contribute to membrane depolarization, but the underlying molecular mechanisms are not fully understood. In this review, we will discuss the historical and physiological significance of the Ca²⁺-activated cation channel, TRPM4, in regulating membrane potential of cerebral artery smooth muscle cells. As a member of the recently described transient receptor potential super family of ion channels, TRPM4 possesses the biophysical properties and upstream cellular signaling and regulatory pathways that establish it as a major physiological player in smooth muscle membrane depolarization.

Molecular mechanism of TRP channels

Transient receptor potential (TRP) channels are cellular sensors for a wide spectrum of physical and chemical stimuli. They are involved in the formation of sight, hearing, touch, smell, taste, temperature, and pain sensation. TRP channels also play fundamental roles in cell signaling and allow the host cell to respond to benign or harmful environmental changes. As TRP channel activation is controlled by very diverse processes and, in many cases, exhibits complex polymodal properties, understanding how each TRP channel responds to its unique forms of activation energy is both crucial and challenging. The past two decades witnessed significant advances in understanding the molecular mechanisms that underlie TRP channels activation. This review focuses on our current understanding of the molecular determinants for TRP channel activation.

Complex N-glycosylation stabilizes surface expression of transient receptor potential melastatin 4b protein

N-glycosylation is important for the function and regulation of ion channels. We examined the role of N-glycosylation of transient receptor potential melastatin (Trpm) 4b, a membrane glycoprotein that regulates calcium influx. Trpm4b was expressed in vivo in all rat tissues examined. In each tissue, Trpm4b had a different molecular mass, between ∼129 and ∼141 kDa, but all reverted to ∼120 kDa following treatment with peptide:N-glycosidase F, consistent with N-glycosylation being the principal form of post-translational modification of Trpm4b in vivo. In six stable isogenic cell lines that express different levels of Trpm4b, two forms were found, high mannose, core-glycosylated and complex, highly glycosylated (HG), with HG-Trpm4b comprising 85% of the total Trpm4b expressed. For both forms, surface expression was directly proportional to the total Trpm4b expressed. Complex N-glycosylation doubled the percentage of Trpm4b at the surface, compared with high mannose N-glycosylation. Mutation of the single N-glycosylation consensus sequence at Asn-988 (Trpm4b-N988Q), located near the pore-forming loop between transmembrane helices 5 and 6, prevented glycosylation, but did not prevent surface expression, impair formation of functional membrane channels, or alter channel conductance. In transfection experiments, the time courses for appearance of HG-Trpm4b and Trpm4b-N988Q on the surface were similar. In experiments with cycloheximide inhibition of protein synthesis, the time course for disappearance of HG-Trpm4b from the surface was much slower than that for Trpm4b-N988Q. We conclude that N-glycosylation is not required for surface expression or channel function, but that complex N-glycosylation plays a crucial role in stabilizing surface expression of Trpm4b.

TRP channels

TRP channels constitute a large superfamily of cation channel forming proteins, all related to the gene product of the transient receptor potential (trp) locus in Drosophila. In mammals, 28 different TRP channel genes have been identified, which exhibit a large variety of functional properties and play diverse cellular and physiological roles. In this article, we provide a brief and systematic summary of expression, function, and (patho)physiological role of the mammalian TRP channels.

The ion channels to cytoskeleton connection as potential mechanism of mechanosensitivity

As biological force-sensing systems mechanosensitive (MS) ion channels present the best example of coupling molecular dynamics of membrane proteins to the mechanics of the surrounding cell membrane. In animal cells MS channels have over the past two decades been very much in focus of mechanotransduction research. In recent years this helped to raise awareness of basic and medical researchers about the role that abnormal MS channels may play in the pathophysiology of diseases, such as cardiac hypertrophy, atrial fibrillation, muscular dystrophy or polycystic kidney disease. To date a large number of MS channels from organisms of diverse phylogenetic origins have been identified at the molecular level; however, the structure of only few of them has been determined. Although their function has extensively been studied in a great variety of cells and tissues by different experimental approaches it is, with exception of bacterial MS channels, very little known about how these channels sense mechanical force and which cellular components may contribute to their function. By focusing on MS channels found in animal cells this article discusses the ways in which the connections between cytoskeleton and ion channels may contribute to mechanosensory transduction in these cells. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Pharmacological and electrophysiological characterization of the human bile acid-sensitive ion channel (hBASIC)

The human bile acid-sensitive ion channel (hBASIC) is a cation channel of the degenerin/epithelial Na(+) channel gene family that is expressed in the intestinal tract and can be activated by bile acids. Here, we show that in addition to its sensitivity for bile acids, hBASIC shares further key features with its rat ortholog: it is blocked by extracellular divalent cations, is inhibited by micromolar concentrations of the diarylamidine diminazene, and activated by millimolar concentrations of flufenamic acid. Furthermore, we demonstrate that two major bile acids present in human bile, chenodeoxycholic acid and deoxycholic acid, activate hBASIC in a synergistic manner. In addition, we determined the single-channel properties of hBASIC in outside-out patch clamp recordings, revealing a single-channel conductance of about 11 pS and a high Na(+) selectivity. Deoxycholic acid activates hBASIC in patch clamp recordings mainly by reducing the single-channel closed time. In summary, we provide a thorough functional characterization of hBASIC.

9-Phenanthrol and flufenamic acid inhibit calcium oscillations in HL-1 mouse cardiomyocytes

It is well established that intracellular calcium ([Ca2+]i) controls the inotropic state of the myocardium, and evidence mounts that a "Ca2+ clock" controls the chronotropic state of the heart. Recent findings describe a calcium-activated nonselective cation channel (NSCCa) in various cardiac preparations sharing hallmark characteristics of the transient receptor potential melastatin 4 (TRPM4). TRPM4 is functionally expressed throughout the heart and has been implicated as a NSCCa that mediates membrane depolarization. However, the functional significance of TRPM4 in regards to Ca2+ signaling and its effects on cellular excitability and pacemaker function remains inconclusive. Here, we show by Fura2 Ca-imaging that pharmacological inhibition of TRPM4 in HL-1 mouse cardiac myocytes by 9-phenanthrol (10 μM) and flufenamic acid (10 and 100 μM) decreases Ca2+ oscillations followed by an overall increase in [Ca2+]i. The latter occurs also in HL-1 cells in Ca(2+)-free solution and after depletion of sarcoplasmic reticulum Ca2+ with thapsigargin (10 μM). These pharmacologic agents also depolarize HL-1 cell mitochondrial membrane potential. Furthermore, by on-cell voltage clamp we show that 9-phenanthrol reversibly inhibits membrane current; by fluorescence immunohistochemistry we demonstrate that HL-1 cells display punctate surface labeling with TRPM4 antibody; and by immunoblotting using this antibody we show these cells express a 130-150 kDa protein, as expected for TRPM4. We conclude that 9-phenanthrol inhibits TRPM4 ion channels in HL-1 cells, which in turn decreases Ca2+ oscillations followed by a compensatory increase in [Ca2+]i from an intracellular store other than the sarcoplasmic reticulum. We speculate that the most likely source is the mitochondrion.

Transient receptor potential melastatin 4 channel controls calcium signals and dental follicle stem cell differentiation

Elevations in the intracellular Ca(2+) concentration are a phenomena commonly observed during stem cell differentiation but cease after the process is complete. The transient receptor potential melastatin 4 (TRPM4) is an ion channel that controls Ca(2+) signals in excitable and nonexcitable cells. However, its role in stem cells remains unknown. The aim of this study was to characterize TRPM4 in rat dental follicle stem cells (DFSCs) and to determine its impact on Ca(2+) signaling and the differentiation process. We identified TRPM4 gene expression in DFSCs, but not TRPM5, a closely related channel with similar function. Perfusion of cells with increasing buffered Ca(2+) resulted in a concentration-dependent activation of currents typical for TRPM4, which were also voltage-dependent and had Na(+) conductivity. Molecular suppression with shRNA decreased channel activity and cell proliferation during osteogenesis but not adipogenesis. As a result, enhanced mineralization and phosphatase enzyme activity were observed during osteoblast formation, although DFSCs failed to differentiate into adipocytes. Furthermore, the normal agonist-induced first and secondary phases of Ca(2+) signals were transformed into a gradual and sustained increase which confirmed the channels' ability to control Ca(2+) signaling. Using whole genome microarray analysis, we identified several genes impacted by TRPM4 during DFSC differentiation. These findings suggest an inhibitory role for TRPM4 on osteogenesis while it appears to be required for adipogenesis. The data also provide a potential link between the Ca(2+) signaling pattern and gene expression during stem cell differentiation.

Oxidative stress-modulated TRPM ion channels in cell dysfunction and pathological conditions in humans

The transient receptor potential melastatin (TRPM) protein family is an extensive group of ion channels expressed in several types of mammalian cells. Many studies have shown that these channels are crucial for performing several physiological functions. Additionally, a large body of evidence indicates that these channels are also involved in numerous human diseases, known as channelopathies. A characteristic event frequently observed during pathological states is the raising in intracellular oxidative agents over reducing molecules, shifting the redox balance and inducing oxidative stress. In particular, three members of the TRPM subfamily, TRPM2, TRPM4 and TRPM7, share the remarkable feature that their activities are modulated by oxidative stress. Because of the increase in oxidative stress, these TRPM channels function aberrantly, promoting the onset and development of diseases. Increases, absences, or modifications in the function of these redox-modulated TRPM channels are associated with cell dysfunction and human pathologies. Therefore, the effect of oxidative stress on ion channels becomes an essential part of the pathogenic mechanism. Thus, oxidative stress-modulated ion channels are more susceptible to generating pathological states than oxidant-independent channels. This review examines the most relevant findings regarding the participation of the oxidative stress-modulated TRPM ion channels, TRPM2, TRPM4, and TRPM7, in human diseases. In addition, the potential roles of these channels as therapeutic tools and targets for drug design are discussed.

TRPM4 channels in smooth muscle function

The melastatin (M) transient receptor potential (TRP) channel TRPM4 is selective for monovalent cations and is activated by high levels of intracellular Ca(2+). TRPM4 is broadly distributed and may be involved in numerous functions, including electrical conduction in the heart, respiratory rhythm, immune response, and secretion of insulin by pancreatic β-cells. The significance of TRPM4 in smooth muscle cell function is reviewed here. Several studies indicate that TRPM4 channels are critically important for pressure-induced cerebral arterial myocyte depolarization and myogenic vasoconstriction as well as autoregulation of cerebral blood flow. Regulation of TRPM4 activity in arterial smooth muscle cells is complex and involves release of Ca(2+) from the sarcoplasmic reticulum through inositol 1,4,5-trisphosphate receptors and translocation of TRPM4 channels to the plasma membrane in response to protein kinase Cδ. TRPM4 is also present in colonic, urinary bladder, aortic, interlobar pulmonary and renal artery, airway, and corpus cavernosum smooth muscle cells, but its significance and regulation in these tissues is less well characterized.

The TRPM4 non-selective cation channel contributes to the mammalian atrial action potential

The TRPM4 calcium-activated non-selective monovalent cation channel has been reported in mammalian atrial cardiomyocytes, but its implication in this tissue remains unknown. We used a combination of pharmacological tools and disruption of the Trpm4 gene in mice to investigate the channel implication in atrial action potential (AP). To search for TRPM4 activity, single channel currents were recorded on freshly isolated atrial cardiomyocytes using the patch-clamp technique. To investigate TRPM4 implication in AP, the transmembrane potential was recorded on the multicellular preparation using intracellular microelectrodes after isolating the mouse atrium, under electrical stimulation (rate=5Hz). Isolated atrial cardiomyocytes from the Trpm4(+/+) mouse expressed a typical TRPM4 current while cardiomyocytes from Trpm4(-/-) mouse did not. The Trpm4(+/+) mouse atrium exhibited AP durations at 50, 70 and 90% repolarization of 8.9±0.5ms, 16.0±1.0ms, and 30.2±1.6ms, respectively. The non-selective cation channel inhibitor flufenamic acid (10(-6) and 10(-5)mol·L(-1)) produced a concentration-dependent decrease in AP duration. Similarly, the TRPM4-inhibitor 9-phenanthrol reversibly reduced the duration of AP with an EC50 at 21×10(-6)mol·L(-1), which is similar to that reported for TRPM4 current inhibition in HEK-293 cells. 9-Phenanthrol had no effect on other AP parameters. The effect of 9-phenanthrol is markedly reduced in the mouse ventricle, which displays only weak expression of the channel. Moreover, atria from Trpm4(-/-) mice exhibited an AP that was 20% shorter than that of atria from littermate control mice, and the effect of 9-phenanthrol on AP was abolished in the Trpm4(-/-) mice. Our results showed that TRPM4 is implicated in the waveform of the atrial action potential. It is thus a potential target for pharmacological approaches against atrial arrhythmias.

TRPM4 cation channel mediates axonal and neuronal degeneration in experimental autoimmune encephalomyelitis and multiple sclerosis

In multiple sclerosis, an inflammatory disease of the central nervous system (CNS), axonal and neuronal loss are major causes for irreversible neurological disability. However, which molecules contribute to axonal and neuronal injury under inflammatory conditions remains largely unknown. Here we show that the transient receptor potential melastatin 4 (TRPM4) cation channel is crucial in this process. TRPM4 is expressed in mouse and human neuronal somata, but it is also expressed in axons in inflammatory CNS lesions in experimental autoimmune encephalomyelitis (EAE) in mice and in human multiple sclerosis tissue. Deficiency or pharmacological inhibition of TRPM4 using the antidiabetic drug glibenclamide resulted in reduced axonal and neuronal degeneration and attenuated clinical disease scores in EAE, but this occurred without altering EAE-relevant immune function. Furthermore, Trpm4(-/-) mouse neurons were protected against inflammatory effector mechanisms such as excitotoxic stress and energy deficiency in vitro. Electrophysiological recordings revealed TRPM4-dependent neuronal ion influx and oncotic cell swelling upon excitotoxic stimulation. Therefore, interference with TRPM4 could translate into a new neuroprotective treatment strategy.

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.

Pathogenic role of store-operated and receptor-operated ca(2+) channels in pulmonary arterial hypertension

Pulmonary circulation is an important circulatory system in which the body brings in oxygen. Pulmonary arterial hypertension (PAH) is a progressive and fatal disease that predominantly affects women. Sustained pulmonary vasoconstriction, excessive pulmonary vascular remodeling, in situ thrombosis, and increased pulmonary vascular stiffness are the major causes for the elevated pulmonary vascular resistance (PVR) in patients with PAH. The elevated PVR causes an increase in afterload in the right ventricle, leading to right ventricular hypertrophy, right heart failure, and eventually death. Understanding the pathogenic mechanisms of PAH is important for developing more effective therapeutic approach for the disease. An increase in cytosolic free Ca²⁺ concentration ([Ca²⁺]_cyt) in pulmonary arterial smooth muscle cells (PASMC) is a major trigger for pulmonary vasoconstriction and an important stimulus for PASMC migration and proliferation which lead to pulmonary vascular wall thickening and remodeling. It is thus pertinent to define the pathogenic role of Ca²⁺ signaling in pulmonary vasoconstriction and PASMC proliferation to develop new therapies for PAH. [Ca²⁺]_cyt in PASMC is increased by Ca²⁺ influx through Ca²⁺ channels in the plasma membrane and by Ca²⁺ release or mobilization from the intracellular stores, such as sarcoplasmic reticulum (SR) or endoplasmic reticulum (ER). There are two Ca²⁺ entry pathways, voltage-dependent Ca²⁺ influx through voltage-dependent Ca²⁺ channels (VDCC) and voltage-independent Ca²⁺ influx through store-operated Ca²⁺ channels (SOC) and receptor-operated Ca²⁺ channels (ROC). This paper will focus on the potential role of VDCC, SOC, and ROC in the development and progression of sustained pulmonary vasoconstriction and excessive pulmonary vascular remodeling in PAH.

Sulfonylurea receptor 1 in central nervous system injury: a focused review

The sulfonylurea receptor 1 (Sur1)-regulated NC(Ca-ATP) channel is a nonselective cation channel that is regulated by intracellular calcium and adenosine triphosphate. The channel is not constitutively expressed, but is transcriptionally upregulated de novo in all cells of the neurovascular unit, in many forms of central nervous system (CNS) injury, including cerebral ischemia, traumatic brain injury (TBI), spinal cord injury (SCI), and subarachnoid hemorrhage (SAH). The channel is linked to microvascular dysfunction that manifests as edema formation and delayed secondary hemorrhage. Also implicated in oncotic cell swelling and oncotic (necrotic) cell death, the channel is a major molecular mechanism of 'accidental necrotic cell death' in the CNS. In animal models of SCI, pharmacological inhibition of Sur1 by glibenclamide, as well as gene suppression of Abcc8, prevents delayed capillary fragmentation and tissue necrosis. In models of stroke and TBI, glibenclamide ameliorates edema, secondary hemorrhage, and tissue damage. In a model of SAH, glibenclamide attenuates the inflammatory response due to extravasated blood. Clinical trials of an intravenous formulation of glibenclamide in TBI and stroke underscore the importance of recent advances in understanding the role of the Sur1-regulated NC(Ca-ATP) channel in acute ischemic, traumatic, and inflammatory injury to the CNS.

The TRPM4 channel controls monocyte and macrophage, but not neutrophil, function for survival in sepsis

A favorable outcome following acute bacterial infection depends on the ability of phagocytic cells to be recruited and properly activated within injured tissues. Calcium (Ca(2+)) is a ubiquitous second messenger implicated in the functions of many cells, but the mechanisms involved in the regulation of Ca(2+) mobilization in hematopoietic cells are largely unknown. The monovalent cation channel transient receptor potential melastatin (TRPM) 4 is involved in the control of Ca(2+) signaling in some hematopoietic cell types, but the role of this channel in phagocytes and its relevance in the control of inflammation remain unexplored. In this study, we report that the ablation of the Trpm4 gene dramatically increased mouse mortality in a model of sepsis induced by cecal ligation and puncture. The lack of the TRPM4 channel affected macrophage population within bacteria-infected peritoneal cavities and increased the systemic level of Ly6C(+) monocytes and proinflammatory cytokine production. Impaired Ca(2+) mobilization in Trpm4(-/-) macrophages downregulated the AKT signaling pathway and the subsequent phagocytic activity, resulting in bacterial overgrowth and translocation to the bloodstream. In contrast, no alteration in the distribution, function, or Ca(2+) mobilization of Trpm4(-/-) neutrophils was observed, indicating that the mechanism controlling Ca(2+) signaling differs among phagocytes. Our results thus show that the tight control of Ca(2+) influx by the TRPM4 channel is critical for the proper functioning of monocytes/macrophages and the efficiency of the subsequent response to infection.

Transient receptor potential melastatin 4 inhibitor 9-phenanthrol abolishes arrhythmias induced by hypoxia and re-oxygenation in mouse ventricle

BACKGROUND AND PURPOSE

Hypoxia and subsequent re-oxygenation are associated with cardiac arrhythmias such as early afterdepolarizations (EADs), which may be partly explained by perturbations in cytosolic calcium concentration. Transient receptor potential melastatin 4 (TRPM4), a calcium-activated non-selective cation channel, is functionally expressed in the heart. Based on its biophysical properties, it is likely to participate in EADs. Hence, modulators of TRPM4 activity may influence arrhythmias. The aim of this study was to investigate the possible anti-arrhythmic effect of 9-phenanthrol, a TRPM4 inhibitor in a murine heart model of hypoxia and re-oxygenation-induced EADs.

EXPERIMENTAL APPROACH

Mouse heart was removed, and the right ventricle was pinned in a superfusion chamber. After a period of normoxia, the preparation was superfused for 2 h with a hypoxic solution and then re-oxygenated. Spontaneous electrical activity was investigated by intracellular microelectrode recordings.

KEY RESULTS

In normoxic conditions, the ventricle exhibited spontaneous action potentials. Application of the hypoxia and re-oxygenation protocol unmasked hypoxia-induced EADs, the occurrence of which increased under re-oxygenation. The frequency of these EADs was reduced by superfusion with either flufenamic acid, a blocker of Ca(2+) -dependent cation channels or with 9-phenanthrol. Superfusion with 9-phenanthrol (10(-5) or 10(-4)  mol·L(-1) ) caused a dramatic dose-dependent abolition of EADs.

CONCLUSIONS AND IMPLICATIONS

Hypoxia and re-oxygenation-induced EADs can be generated in the mouse heart model. 9-Phenanthrol abolished EADs, which strongly suggests the involvement of TRPM4 in the generation of EAD. This identifies non-selective cation channels inhibitors as new pharmacological candidates in the treatment of arrhythmias.

On potential interactions between non-selective cation channel TRPM4 and sulfonylurea receptor SUR1

The sulfonylurea receptor SUR1 associates with Kir6.2 or Kir6.1 to form K(ATP) channels, which link metabolism to excitability in multiple cell types. The strong physical coupling of SUR1 with Kir6 subunits appears exclusive, but recent studies argue that SUR1 also modulates TRPM4, a member of the transient receptor potential family of non-selective cation channels. It has been reported that, following stroke, brain, or spinal cord injury, SUR1 is increased in neurovascular cells at the site of injury. This is accompanied by up-regulation of a non-selective cation conductance with TRPM4-like properties and apparently sensitive to sulfonylureas, leading to the postulation that post-traumatic non-selective cation currents are determined by TRPM4/SUR1 channels. To investigate the mechanistic hypothesis for the coupling between TRPM4 and SUR1, we performed electrophysiological and FRET studies in COSm6 cells expressing TRPM4 channels with or without SUR1. TRPM4-mediated currents were Ca(2+)-activated, voltage-dependent, underwent desensitization, and were inhibited by ATP but were insensitive to glibenclamide and tolbutamide. These properties were not affected by cotransfection with SUR1. When the same SUR1 was cotransfected with Kir6.2, functional K(ATP) channels were formed. In cells cotransfected with Kir6.2, SUR1, and TRPM4, we measured K(ATP)-mediated K(+) currents and Ca(2+)-activated, sulfonylurea-insensitive Na(+) currents in the same patch, further showing that SUR1 controls K(ATP) channel activity but not TRPM4 channels. FRET signal between fluorophore-tagged TRPM4 subunits was similar to that between Kir6.2 and SUR1, whereas there was no detectable FRET efficiency between TRPM4 and SUR1. Our data suggest that functional or structural association of TRPM4 and SUR1 is unlikely.

Endogenous cytosolic Ca(2+) buffering is necessary for TRPM4 activity in cerebral artery smooth muscle cells

The melastatin transient receptor potential (TRP) channel, TRPM4, is a critical regulator of smooth muscle membrane potential and arterial tone. Activation of the channel is Ca(2+)-dependent, but prolonged exposures to high global Ca(2+) causes rapid inactivation under conventional whole-cell patch clamp conditions. Using amphotericin B perforated whole cell patch clamp electrophysiology, which minimally disrupts cytosolic Ca(2+) dynamics, we recently showed that Ca(2+) released from 1,2,5-triphosphate receptors (IP(3)R) on the sarcoplasmic reticulum (SR) activates TRPM4 channels, producing sustained transient inward cation currents (TICCs). Thus, Ca(2+)-dependent inactivation of TRPM4 may not be inherent to the channel itself but rather is a result of the recording conditions. We hypothesized that under conventional whole-cell configurations, loss of intrinsic cytosolic Ca(2+) buffering following cell dialysis contributes to inactivation of TRPM4 channels. With the inclusion of the Ca(2+) buffers ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA, 10mM) or bis-ethane-N,N,N',N'-tetraacetic acid (BAPTA, 0.1mM) in the pipette solution, we mimic endogenous Ca(2+) buffering and record novel, sustained whole-cell TICC activity from freshly-isolated cerebral artery myocytes. Biophysical properties of TICCs recorded under perforated and whole-cell patch clamp were nearly identical. Furthermore, whole-cell TICC activity was reduced by the selective TRPM4 inhibitor, 9-phenanthrol, and by siRNA-mediated knockdown of TRPM4. When a higher concentration (10mM) of BAPTA was included in the pipette solution, TICC activity was disrupted, suggesting that TRPM4 channels on the plasma membrane and IP(3)R on the SR are closely opposed but not physically coupled, and that endogenous Ca(2+) buffer proteins play a critical role in maintaining TRPM4 channel activity in native cerebral artery smooth muscle cells.