Functional expression of the TRPM4 cationic current in ventricular cardiomyocytes from spontaneously hypertensive rats

The mechanisms of exercise improving cardiovascular function by stimulating Piezo1 and TRP ion channels: a systemic review

Mechanosensitive ion channels are widely distributed in the heart, lung, bladder and other tissues, and plays an important role in exercise-induced cardiovascular function promotion. By reviewing the PubMed databases, the results were summarized using the terms "Exercise/Sport", "Piezo1", "Transient receptor potential (TRP)" and "Cardiovascular" as the keywords, 124-related papers screened were sorted and reviewed. The results showed that: (1) Piezo1 and TRP channels play an important role in regulating blood pressure and the development of cardiovascular diseases such as atherosclerosis, myocardial infarction, and cardiac fibrosis; (2) Exercise promotes cardiac health, inhibits the development of pathological heart to heart failure, regulating the changes in the characterization of Piezo1 and TRP channels; (3) Piezo1 activates downstream signaling pathways with very broad pathways, such as AKT/eNOS, NF-κB, p38MAPK and HIPPO-YAP signaling pathways. Piezo1 and Irisin regulate nuclear localization of YAP and are hypothesized to act synergistically to regulate tissue mechanical properties of the cardiovascular system and (4) The cardioprotective effects of exercise through the TRP family are mostly accomplished through Ca2+ and involve many signaling pathways. TRP channels exert their important cardioprotective effects by reducing the TRPC3-Nox2 complex and mediating Irisin-induced Ca2+ influx through TRPV4. It is proposed that exercise stimulates the mechanosensitive cation channel Piezo1 and TRP channels, which exerts cardioprotective effects. The activation of Piezo1 and TRP channels and their downstream targets to exert cardioprotective function by exercise may provide a theoretical basis for the prevention of cardiovascular diseases and the rehabilitation of clinical patients.

Uncoupling cytosolic calcium from membrane voltage by transient receptor potential melastatin 4 channel (TRPM4) modulation: A novel strategy to treat ventricular arrhythmias

The current antiarrhythmic paradigm is mainly centered around modulating membrane voltage. However, abnormal cytosolic calcium (Ca2+) signaling, which plays an important role in driving membrane voltage, has not been targeted for therapeutic purposes in arrhythmogenesis. There is clear evidence for bidirectional coupling between membrane voltage and intracellular Ca2+. Cytosolic Ca2+ regulates membrane voltage through Ca2+-sensitive membrane currents. As a component of Ca2+-sensitive currents, Ca2+-activated nonspecific cationic current through the TRPM4 (transient receptor potential melastatin 4) channel plays a significant role in Ca2+-driven changes in membrane electrophysiology. In myopathic and ischemic ventricles, upregulation and/or enhanced activity of this current is associated with the generation of afterdepolarization (both early and delayed), reduction of repolarization reserve, and increased propensity to ventricular arrhythmias. In this review, we describe a novel concept for the management of ventricular arrhythmias in the remodeled ventricle based on mechanistic concepts from experimental studies, by uncoupling the Ca2+-induced changes in membrane voltage by inhibition of this TRPM4-mediated current.

Identification of RNA reads encoding different channels in isolated rat ventricular myocytes and the effect of cell stretching on L-type Ca2+current

BACKGROUND

The study aimed to identify transcripts of specific ion channels in rat ventricular cardiomyocytes and determine their potential role in the regulation of ionic currents in response to mechanical stimulation. The gene expression levels of various ion channels in freshly isolated rat ventricular cardiomyocytes were investigated using the RNA-seq technique. We also measured changes in current through CaV1.2 channels under cell stretching using the whole-cell patch-clamp method.

RESULTS

Among channels that showed mechanosensitivity, significant amounts of TRPM7, TRPC1, and TRPM4 transcripts were found. We suppose that the recorded L-type Ca2+ current is probably expressed through CaV1.2. Furthermore, stretching cells by 6, 8, and 10 μm, which increases ISAC through the TRPM7, TRPC1, and TRPM4 channels, also decreased ICa,L through the CaV1.2 channels in K+ in/K+ out, Cs+ in/K+ out, K+ in/Cs+ out, and Cs+ in/Cs+ out solutions. The application of a nonspecific ISAC blocker, Gd3+, during cell stretching eliminated ISAC through nonselective cation channels and ICa,L through CaV1.2 channels. Since the response to Gd3+ was maintained in Cs+ in/Cs+ out solutions, we suggest that voltage-gated CaV1.2 channels in the ventricular myocytes of adult rats also exhibit mechanosensitive properties.

CONCLUSIONS

Our findings suggest that TRPM7, TRPC1, and TRPM4 channels represent stretch-activated nonselective cation channels in rat ventricular myocytes. Probably the CaV1.2 channels in these cells exhibit mechanosensitive properties. Our results provide insight into the molecular mechanisms underlying stretch-induced responses in rat ventricular myocytes, which may have implications for understanding cardiac physiology and pathophysiology.

Knockdown of the TRPM4 channel alters cardiac electrophysiology and hemodynamics in a sex- and age-dependent manner in mice

TRPM4 is a calcium-activated, voltage-modulated, nonselective ion channel widely expressed in various cells and tissues. TRPM4 regulates the influx of sodium ions, thus playing a role in regulating the membrane potential. In the heart, TRPM4 is expressed in both cardiomyocytes and cells of the conductive pathways. Clinical studies have linked TRPM4 mutations to several cardiac disorders. While data from experimental studies have demonstrated TRPM4's functional significance in cardiac physiology, its exact roles in the heart have remained unclear. In this study, we investigated the role of TRPM4 in cardiac physiology in a newly generated Trpm4 knockdown mouse model. Male and female Trpm4 knockdown (Trpm4-/- ) and wild-type mice of different ages (5- to 12- week-old (young) and 24-week-old or more (adult)) were characterized using a multimodal approach, encompassing surface electrocardiograms (ECG), echocardiography recordings, ex vivo ECGs in isolated heart, endocardial mappings, Western blots, and mRNA quantifications. The assessment of cardiac electrophysiology by surface ECGs revealed no significant differences between wild-type and Trpm4-/- young (5- to 12-week-old) mice of either sex. Above 24 weeks of age, adult male Trpm4-/- mice showed reduced heart rate and increased heart rate variability. Echocardiography revealed that only adult male Trpm4-/- mice exhibited slight left ventricular hypertrophic alterations compared to controls, illustrated by alterations of the mitral valve pressure halftime, the mitral valve E/A ratio, the isovolumetric relaxation time, and the mitral valve deceleration. In addition, an assessment of the right ventricular systolic function by scanning the pulmonary valve highlighted an alteration in pulmonary valve peak velocity and pressure in adult male Trpm4-/- mice. Endocardial mapping recordings showed that applying 5 μM of the new TRPM4 inhibitor NBA triggered a third-degree atrioventricular block on 40% of wild-type hearts. These results confirm the key role of TRPM4 in the proper structure and electrical function of the heart. It also reveals differences between male and female animals that have never been reported. In addition, the investigation of the effects of NBA on heart function confirms the role of TRPM4 in atrioventricular conduction.

The Role of TRPM4 in Cardiac Electrophysiology and Arrhythmogenesis

The transient receptor potential melastatin 4 (TRPM4) channel is a non-selective cation channel that activates in response to increased intracellular Ca²⁺ levels but does not allow Ca²⁺ to pass through directly. It plays a crucial role in regulating diverse cellular functions associated with intracellular Ca²⁺ homeostasis/dynamics. TRPM4 is widely expressed in the heart and is involved in various physiological and pathological processes therein. Specifically, it has a significant impact on the electrical activity of cardiomyocytes by depolarizing the membrane, presumably via Na⁺ loading. The TRPM4 channel likely contributes to the development of cardiac arrhythmias associated with specific genetic backgrounds and cardiac remodeling. This short review aims to overview what is known so far about the TRPM4 channel in cardiac electrophysiology and arrhythmogenesis, highlighting its potential as a novel therapeutic target to effectively prevent and treat cardiac arrhythmias.

Electrical Remodelling in Cardiac Disease

The human heart responds to various diseases with structural, mechanical, and electrical remodelling processes [...].

Emerging role of transient receptor potential (TRP) ion channels in cardiac fibroblast pathophysiology

Cardiac fibroblasts make up a major proportion of non-excitable cells in the heart and contribute to the cardiac structural integrity and maintenance of the extracellular matrix. During myocardial injury, fibroblasts can be activated to trans-differentiate into myofibroblasts, which secrete extracellular matrix components as part of healing, but may also induce cardiac fibrosis and pathological cardiac structural and electrical remodeling. The mechanisms regulating such cellular processes still require clarification, but the identification of transient receptor potential (TRP) channels in cardiac fibroblasts could provide further insights into the fibroblast-related pathophysiology. TRP proteins belong to a diverse superfamily, with subgroups such as the canonical (TRPC), vanilloid (TRPV), melastatin (TRPM), ankyrin (TRPA), polycystin (TRPP), and mucolipin (TRPML). Several TRP proteins form non-selective channels that are permeable to cations like Na+ and Ca2+ and are activated by various chemical and physical stimuli. This review highlights the role of TRP channels in cardiac fibroblasts and the possible underlying signaling mechanisms. Changes in the expression or activity of TRPs such as TRPCs, TRPVs, TRPMs, and TRPA channels modulate cardiac fibroblasts and myofibroblasts, especially under pathological conditions. Such TRPs contribute to cardiac fibroblast proliferation and differentiation as well as to disease conditions such as cardiac fibrosis, atrial fibrillation, and fibroblast metal toxicity. Thus, TRP channels in fibroblasts represent potential drug targets in cardiac disease.

Flufenamic acid improves survival and neurologic outcome after successful cardiopulmonary resuscitation in mice

Background

Brain injury is the main cause of high mortality and disability after successful cardiopulmonary resuscitation (CPR) from sudden cardiac arrest (CA). The transient receptor potential M4 (TRPM4) channel is a novel target for ameliorating blood–brain barrier (BBB) disruption and neuroinflammation. Herein, we tested whether flufenamic acid (FFA), which is reported to block TRPM4 with high potency, could confer neuroprotection against brain injury secondary to CA/CPR and whether its action was exerted by blocking the TRPM4 channel.

Methods

Wild-type (WT) and Trpm4 knockout (Trpm4^−/−) mice subjected to 10-min CA/CPR were randomized to receive FFA or vehicle once daily. Post-CA/CPR brain injuries including neurologic deficits, survival rate, histological damage, edema formation, BBB destabilization and neuroinflammation were assessed.

Results

In WT mice subjected to CA/CPR, FFA was effective in improving survival and neurologic outcome, reducing neuropathological injuries, attenuating brain edema, lessening the leakage of IgG and Evans blue dye, restoring tight junction protein expression and promoting microglia/macrophages from the pro-inflammatory subtype toward the anti-inflammatory subtype. In comparison to WT mice, Trpm4^−/− mice exhibited less neurologic deficiency, milder histological impairment, more BBB integrity and more anti-inflammatory microglia/macrophage polarization. As expected, FFA did not provide a benefit of superposition compared with vehicle in the Trpm4^−/− mice after CA/CPR.

Conclusions

FFA mitigates BBB breach and modifies the functional status of microglia/macrophages, thereby improving survival and neurologic deficits following CA/CPR. The neuroprotective effects occur at least partially by interfering with the TRPM4 channel in the neurovascular unit. These results indicate the significant clinical potential of FFA to improve the prognosis for CA victims who are successfully resuscitated.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02571-2.

Emergent Temporal Signaling in Human Trabecular Meshwork Cells: Role of TRPV4-TRPM4 Interactions

Trabecular meshwork (TM) cells are phagocytic cells that employ mechanotransduction to actively regulate intraocular pressure. Similar to macrophages, they express scavenger receptors and participate in antigen presentation within the immunosuppressive milieu of the anterior eye. Changes in pressure deform and compress the TM, altering their control of aqueous humor outflow but it is not known whether transducer activation shapes temporal signaling. The present study combines electrophysiology, histochemistry and functional imaging with gene silencing and heterologous expression to gain insight into Ca²⁺ signaling downstream from TRPV4 (Transient Receptor Potential Vanilloid 4), a stretch-activated polymodal cation channel. Human TM cells respond to the TRPV4 agonist GSK1016790A with fluctuations in intracellular Ca²⁺ concentration ([Ca²⁺]_i) and an increase in [Na⁺]_i. [Ca²⁺]_i oscillations coincided with monovalent cation current that was suppressed by BAPTA, Ruthenium Red and the TRPM4 (Transient Receptor Potential Melastatin 4) channel inhibitor 9-phenanthrol. TM cells expressed TRPM4 mRNA, protein at the expected 130-150 kDa and showed punctate TRPM4 immunoreactivity at the membrane surface. Genetic silencing of TRPM4 antagonized TRPV4-evoked oscillatory signaling whereas TRPV4 and TRPM4 co-expression in HEK-293 cells reconstituted the oscillations. Membrane potential recordings suggested that TRPM4-dependent oscillations require release of Ca²⁺ from internal stores. 9-phenanthrol did not affect the outflow facility in mouse eyes and eyes from animals lacking TRPM4 had normal intraocular pressure. Collectively, our results show that TRPV4 activity initiates dynamic calcium signaling in TM cells by stimulating TRPM4 channels and intracellular Ca²⁺ release. It is possible that TRPV4-TRPM4 interactions downstream from the tensile and compressive impact of intraocular pressure contribute to homeostatic regulation and pathological remodeling within the conventional outflow pathway.

Arrhythmogenesis in the aged heart following ischaemia-reperfusion: role of transient receptor potential vanilloid 4

AIMS

Cardiomyocyte Ca2+ homoeostasis is altered with ageing and predisposes the heart to Ca2+ intolerance and arrhythmia. Transient receptor potential vanilloid 4 (TRPV4) is an osmotically activated cation channel with expression in cardiomyocytes of the aged heart. The objective of this study was to examine the role of TRPV4 in Ca2+ handling and arrhythmogenesis following ischaemia-reperfusion (I/R), a pathological scenario associated with osmotic stress.

METHODS AND RESULTS

Cardiomyocyte membrane potential was monitored prior to and following I/R in Langendorff-perfused hearts of Aged (19-28 months) male and female C57BL/6 mice ± TRPV4 inhibition (1 μM HC067047, HC). Diastolic resting membrane potential was similar between Aged and Aged HC at baseline, but following I/R Aged exhibited depolarized diastolic membrane potential vs. Aged HC. The effects of TRPV4 on cardiomyocyte Ca2+ signalling following I/R were examined in isolated hearts of Aged cardiac-specific GCaMP6f mice (±HC) using high-speed confocal fluorescence microscopy, with cardiomyocytes of Aged exhibiting an increased incidence of pro-arrhythmic Ca2+ signalling vs. Aged HC. In the isolated cell environment, cardiomyocytes of Aged responded to sustained hypoosmotic stress (250mOsm) with an increase in Ca2+ transient amplitude (fluo-4) and higher incidence of pro-arrhythmic diastolic Ca2+ signals vs. Aged HC. Intracardiac electrocardiogram measurements in isolated hearts following I/R revealed an increased arrhythmia incidence, an accelerated time to ventricular arrhythmia, and increased arrhythmia score in Aged vs. Aged HC. Aged exhibited depolarized resting membrane potential, increased pro-arrhythmic diastolic Ca2+ signalling, and greater incidence of arrhythmia when compared with Young (3-5 months).

CONCLUSION

TRPV4 contributes to pro-arrhythmic cardiomyocyte Ca2+ signalling, electrophysiological abnormalities, and ventricular arrhythmia in the aged mouse heart.

Unraveling the Cardiac Effects Induced by Carvacrol in Spontaneously Hypertensive Rats: Involvement of Transient Receptor Potential Melastatin Subfamily 4 and 7 Channels

ABSTRACT

Accumulating evidence indicates that transient receptor potential (TRP) channels are involved in the pathophysiological process in the heart, and monoterpenes, such as carvacrol, are able to modulate these channels activity. In this article, our purpose was to evaluate the direct cardiac effect of carvacrol on the contractility of cardiomyocytes and isolated right atria from spontaneously hypertensive and Wistar Kyoto rats. In this way, in vitro experiments were used to evaluate the ventricular cardiomyocytes contractility and the Ca2+ transient measuring, in addition to heart rhythm in the right atria. The role of TRPM channels in carvacrol-mediated cardiac activities was also investigated. The results demonstrated that carvacrol induced a significant reduction in ventricular cell contractility, without changes in transient Ca2+. In addition, carvacrol promoted a significant negative chronotropic response in spontaneously hypertensive and Wistar Kyoto rats' atria. Selective blockage of TRPM channels suggests the involvement of TRP melastatin subfamily 2 (TRPM2), TRPM4, and TRPM7 in the carvacrol-mediated cardiac effects. In silico studies were conducted to further investigate the putative role of TRPM4 in carvacrol-mediated cardiac action. FTMap underscores a conserved pocket in both TRPM4 and TRPM7, revealing a potential carvacrol binding site, and morphological similarity analysis demonstrated that carvacrol shares a more than 85% similarity to 9-phenanthrol. Taken together, these results suggest that carvacrol has direct cardiac actions, leading to reduced cellular contractility and inducing a negative chronotropic effect, which may be related to TRPM7 and TRPM4 modulation.

A Central Role for TRPM4 in Ca2+-Signal Amplification and Vasoconstriction

Transient receptor potential melastatin-4 (TRPM4) is activated by an increase in intracellular Ca2+ concentration and is expressed on smooth muscle cells (SMCs). It is implicated in the myogenic constriction of cerebral arteries. We hypothesized that TRPM4 has a general role in intracellular Ca2+ signal amplification in a wide range of blood vessels. TRPM4 function was tested with the TRPM4 antagonist 9-phenanthrol and the TRPM4 activator A23187 on the cardiovascular responses of the rat, in vivo and in isolated basilar, mesenteric, and skeletal muscle arteries. TRPM4 inhibition by 9-phenanthrol resulted in hypotension and a decreased heart rate in the rat. TRPM4 inhibition completely antagonized myogenic tone development and norepinephrine-evoked vasoconstriction, and depolarization (high extracellular KCl concentration) evoked vasoconstriction in a wide range of peripheral arteries. Vasorelaxation caused by TRPM4 inhibition was accompanied by a significant decrease in intracellular Ca2+ concentration, suggesting an inhibition of Ca2+ signal amplification. Immunohistochemistry confirmed TRPM4 expression in the smooth muscle cells of the peripheral arteries. Finally, TRPM4 activation by the Ca2+ ionophore A23187 was competitively inhibited by 9-phenanthrol. In summary, TRPM4 was identified as an essential Ca2+-amplifying channel in peripheral arteries, contributing to both myogenic tone and agonist responses. These results suggest an important role for TRPM4 in the circulation. The modulation of TRPM4 activity may be a therapeutic target for hypertension. Furthermore, the Ca2+ ionophore A23187 was identified as the first high-affinity (nanomolar) direct activator of TRPM4, acting on the 9-phenanthrol binding site.

Pharmacological Modulation and (Patho)Physiological Roles of TRPM4 Channel—Part 1: Modulation of TRPM4

Transient receptor potential melastatin 4 is a unique member of the TRPM protein family and, similarly to TRPM5, is Ca²⁺-sensitive and permeable to monovalent but not divalent cations. It is widely expressed in many organs and is involved in several functions by regulating the membrane potential and Ca²⁺ homeostasis in both excitable and non-excitable cells. This part of the review discusses the pharmacological modulation of TRPM4 by listing, comparing, and describing both endogenous and exogenous activators and inhibitors of the ion channel. Moreover, other strategies used to study TRPM4 functions are listed and described. These strategies include siRNA-mediated silencing of TRPM4, dominant-negative TRPM4 variants, and anti-TRPM4 antibodies. TRPM4 is receiving more and more attention and is likely to be the topic of research in the future.

Pharmacological Modulation and (Patho)Physiological Roles of TRPM4 Channel-Part 2: TRPM4 in Health and Disease

Transient receptor potential melastatin 4 (TRPM4) is a unique member of the TRPM protein family and, similarly to TRPM5, is Ca²⁺ sensitive and permeable for monovalent but not divalent cations. It is widely expressed in many organs and is involved in several functions; it regulates membrane potential and Ca²⁺ homeostasis in both excitable and non-excitable cells. This part of the review discusses the currently available knowledge about the physiological and pathophysiological roles of TRPM4 in various tissues. These include the physiological functions of TRPM4 in the cells of the Langerhans islets of the pancreas, in various immune functions, in the regulation of vascular tone, in respiratory and other neuronal activities, in chemosensation, and in renal and cardiac physiology. TRPM4 contributes to pathological conditions such as overactive bladder, endothelial dysfunction, various types of malignant diseases and central nervous system conditions including stroke and injuries as well as in cardiac conditions such as arrhythmias, hypertrophy, and ischemia-reperfusion injuries. TRPM4 claims more and more attention and is likely to be the topic of research in the future.

Electrophysiological Effects of the Transient Receptor Potential Melastatin 4 Channel Inhibitor (4-Chloro-2-(2-chlorophenoxy)acetamido) Benzoic Acid (CBA) in Canine Left Ventricular Cardiomyocytes

Transient receptor potential melastatin 4 (TRPM4) plays an important role in many tissues, including pacemaker and conductive tissues of the heart, but much less is known about its electrophysiological role in ventricular myocytes. Our earlier results showed the lack of selectivity of 9-phenanthrol, so CBA ((4-chloro-2-(2-chlorophenoxy)acetamido) benzoic acid) was chosen as a new, potentially selective inhibitor. Goal: Our aim was to elucidate the effect and selectivity of CBA in canine left ventricular cardiomyocytes and to study the expression of TRPM4 in the canine heart. Experiments were carried out in enzymatically isolated canine left ventricular cardiomyocytes. Ionic currents were recorded with an action potential (AP) voltage-clamp technique in whole-cell configuration at 37 °C. An amount of 10 mM BAPTA was used in the pipette solution to exclude the potential activation of TRPM4 channels. AP was recorded with conventional sharp microelectrodes. CBA was used in 10 µM concentrations. Expression of TRPM4 protein in the heart was studied by Western blot. TRPM4 protein was expressed in the wall of all four chambers of the canine heart as well as in samples prepared from isolated left ventricular cells. CBA induced an approximately 9% reduction in AP duration measured at 75% and 90% of repolarization and decreased the short-term variability of APD₉₀. Moreover, AP amplitude was increased and the maximal rates of phase 0 and 1 were reduced by the drug. In AP clamp measurements, CBA-sensitive current contained a short, early outward and mainly a long, inward current. Transient outward potassium current (I_to) and late sodium current (I_Na,L) were reduced by approximately 20% and 47%, respectively, in the presence of CBA, while L-type calcium and inward rectifier potassium currents were not affected. These effects of CBA were largely reversible upon washout. Based on our results, the CBA induced reduction of phase-1 slope and the slight increase of AP amplitude could have been due to the inhibition of I_to. The tendency for AP shortening can be explained by the inhibition of inward currents seen in AP-clamp recordings during the plateau phase. This inward current reduced by CBA is possibly I_Na,L, therefore, CBA is not entirely selective for TRPM4 channels. As a consequence, similarly to 9-phenanthrol, it cannot be used to test the contribution of TRPM4 channels to cardiac electrophysiology in ventricular cells, or at least caution must be applied.

Transient receptor potential vanilloid-4 contributes to stretch-induced hypercontractility and time-dependent dysfunction in the aged heart

AIMS

Cardiovascular disease remains the greatest cause of mortality in Americans over 65. The stretch-activated transient receptor potential vanilloid-4 (TRPV4) ion channel is expressed in cardiomyocytes of the aged heart. This investigation tests the hypothesis that TRPV4 alters Ca2+ handling and cardiac function in response to increased ventricular preload and cardiomyocyte stretch.

METHODS AND RESULTS

Left ventricular maximal pressure (PMax) was monitored in isolated working hearts of Aged (24-27 months) mice following preload elevation from 5 to 20mmHg, with and without TRPV4 antagonist HC067047 (HC, 1 µmol/L). In preload responsive hearts, PMax prior to and immediately following preload elevation (i.e. Frank-Starling response) was similar between Aged and Aged+HC. Within 1 min following preload elevation, Aged hearts demonstrated secondary PMax augmentation (Aged>Aged+HC) suggesting a role for stretch-activated TRPV4 in cardiac hypercontractility. However, after 20 min at 20 mmHg Aged exhibited depressed PMax (Aged<Aged+HC) suggestive of TRPV4-dependent contractile dysfunction with sustained stretch. To examine stretch-induced Ca2+ homeostasis at the single-cell level, isolated cardiomyocytes were stretched 10-15% of slack length while measuring intracellular Ca2+ with fura-2. Uniaxial longitudinal stretch increased intracellular Ca2+ levels and triggered Ca2+ overload and terminal cellular contracture in Aged, but not Aged+HC. Preload elevation in hearts of young/middle-age (3-12 months) mice produced an initial PMax increase (Frank-Starling response) without secondary PMax augmentation, and cardiomyocyte stretch did not affect intracellular Ca2+ levels. Hearts of transgenic mice with cardiac-specific TRPV4 expression exhibited PMax similar to 3- to 12-month control mice prior to and immediately following preload elevation but displayed secondary PMax augmentation. Cardiomyocytes of mice with transgenic TRPV4 expression were highly sensitive to mechanical stimulation and exhibited elevated Ca2+ levels, Ca2+ overload, and terminal contracture upon cellular attachment and stretch.

CONCLUSION

TRPV4 contributes to a stretch-induced increase in cardiomyocyte Ca2+ and cardiac hypercontractility, yet sustained stretch leads to cardiomyocyte Ca2+ overload and contractile dysfunction in the aged heart.

The Ca2+-activated cation channel TRPM4 is a positive regulator of pressure overload-induced cardiac hypertrophy

Pathological left ventricular hypertrophy (LVH) occurs in response to pressure overload and remains the single most important clinical predictor of cardiac mortality. The molecular pathways in the induction of pressure overload LVH are potential targets for therapeutic intervention. Current treatments aim to remove the pressure overload stimulus for LVH, but do not completely reverse adverse cardiac remodelling. Although numerous molecular signalling steps in the induction of LVH have been identified, the initial step by which mechanical stretch associated with cardiac pressure overload is converted into a chemical signal that initiates hypertrophic signalling remains unresolved. In this study, we show that selective deletion of transient receptor potential melastatin 4 (TRPM4) channels in mouse cardiomyocytes results in an approximately 50% reduction in the LVH induced by transverse aortic constriction. Our results suggest that TRPM4 channel is an important component of the mechanosensory signalling pathway that induces LVH in response to pressure overload and represents a potential novel therapeutic target for the prevention of pathological LVH.

An Arrhythmic Mutation E7K Facilitates TRPM4 Channel Activation via Enhanced PIP2 Interaction

A Ca²⁺-activated monovalent cation-selective TRPM4 channel is abundantly expressed in the heart. Recently, a single gain-of-function mutation identified in the distal N-terminus of the human TRPM4 channel (Glu⁵ to Lys⁵; E7K) was found to be arrhythmogenic because of enhanced cell membrane expression. In this study, we conducted detailed analyses of this mutant channel from more functional aspects, in comparison with its wild type (WT). In an expression system, intracellular application of a short soluble PIP₂ (diC₈PIP₂) restored the single-channel activities of both WT and E7K, which had quickly faded after membrane excision. The potency (K_d) of diC₈PIP₂ for this recovery was stronger in E7K than its WT (1.44 vs. 2.40 μM). FRET-based PIP₂ measurements combined with the Danio rerio voltage-sensing phosphatase (DrVSP) and patch clamping revealed that lowering the endogenous PIP₂ level by DrVSP activation reduced the TRPM4 channel activity. This effect was less prominent in E7K than its WT (apparent K_d values estimated from DrVSP-mediated PIP₂ depletion: 0.97 and 1.06 μM, respectively), being associated with the differential PIP₂-mediated modulation of voltage dependence. Moreover, intracellular perfusion of short N-terminal polypeptides containing either the ‘WT’ or ‘E7K’ sequences respectively attenuated the TRPM4 channel activation at whole-cell and single-channel levels, but in both configurations, the E7K polypeptide exerted greater inhibitory effects. These results collectively suggest that N-terminal interaction with endogenous PIP₂ is essential for the TRPM4 channel to function, the extent of which may be abnormally strengthened by the E7K mutation through modulating voltage-dependent activation. The altered PIP₂ interaction may account for the arrhythmogenic potential of this mutation.

Systematic Understanding of Pathophysiological Mechanisms of Oxidative Stress-Related Conditions-Diabetes Mellitus, Cardiovascular Diseases, and Ischemia-Reperfusion Injury

Reactive oxygen species (ROS) plays a role in intracellular signal transduction under physiological conditions while also playing an essential role in diseases such as hypertension, ischemic heart disease, and diabetes, as well as in the process of aging. The influence of ROS has some influence on the frequent occurrence of cardiovascular diseases (CVD) in diabetic patients. In this review, we considered the pathophysiological relationship between diabetes and CVD from the perspective of ROS. In addition, considering organ damage due to ROS elevation during ischemia-reperfusion, we discussed heart and lung injuries. Furthermore, we have focused on the transient receptor potential (TRP) channels and L-type calcium channels as molecular targets for ROS in ROS-induced tissue damages and have discussed about the pathophysiological mechanism of the injury.

Deletion of Trpm4 Alters the Function of the Nav1.5 Channel in Murine Cardiac Myocytes

Transient receptor potential melastatin member 4 (TRPM4) encodes a Ca²⁺-activated, non-selective cation channel that is functionally expressed in several tissues, including the heart. Pathogenic mutants in TRPM4 have been reported in patients with inherited cardiac diseases, including conduction blockage and Brugada syndrome. Heterologous expression of mutant channels in cell lines indicates that these mutations can lead to an increase or decrease in TRPM4 expression and function at the cell surface. While the expression and clinical variant studies further stress the importance of TRPM4 in cardiac function, the cardiac electrophysiological phenotypes in Trpm4 knockdown mouse models remain incompletely characterized. To study the functional consequences of Trpm4 deletion on cardiac electrical activity in mice, we performed perforated-patch clamp and immunoblotting studies on isolated atrial and ventricular cardiac myocytes and surfaces, as well as on pseudo- and intracardiac ECGs, either in vivo or in Langendorff-perfused explanted mouse hearts. We observed that TRPM4 is expressed in atrial and ventricular cardiac myocytes and that deletion of Trpm4 unexpectedly reduces the peak Na⁺ currents in myocytes. Hearts from Trpm4^−/− mice presented increased sensitivity towards mexiletine, a Na⁺ channel blocker, and slower intraventricular conduction, consistent with the reduction of the peak Na⁺ current observed in the isolated cardiac myocytes. This study suggests that TRPM4 expression impacts the Na⁺ current in murine cardiac myocytes and points towards a novel function of TRPM4 regulating the Na_v1.5 function in murine cardiac myocytes.

Upregulation of transient receptor potential melastatin 4 (TRPM4) in ventricular fibroblasts from heart failure patients

The transient receptor potential melastatin 4 (TRPM4) is a Ca2+-activated nonselective monovalent cation channel belonging to the TRP channel superfamily. TRPM4 is widely expressed in various tissues and most abundantly expressed in the heart. TRPM4 plays a critical role in cardiac conduction. Patients carrying a gain-of-function or loss-of-function mutation of TRPM4 display impaired cardiac conduction. Knockout or over-expression of TRPM4 in mice recapitulates conduction defects in patients. Moreover, recent studies have indicated that TRPM4 plays a role in hypertrophy and heart failure. Whereas the role of TRPM4 mediated by cardiac myocytes has been well investigated, little is known about TRPM4 and its role in cardiac fibroblasts. Here we show that in human left ventricular fibroblasts, TRPM4 exhibits typical Ca2+-activation characteristics, linear current-voltage (I-V) relation, and monovalent permeability. TRPM4 currents recorded in fibroblasts from heart failure patients (HF) are more than 2-fold bigger than those from control individuals (CTL). The enhanced functional TRPM4 in HF is not resulted from changed channel properties, as TRPM4 currents from both HF and CTL fibroblasts demonstrate similar sensitivity to intracellular calcium activation and extracellular 9-phenanthrol (9-phen) blockade. Consistent with enhanced TRPM4 activity, the protein level of TRPM4 is about 2-fold higher in HF than that of CTL hearts. Moreover, TRPM4 current in CTL fibroblasts is increased after 24 hours of TGFβ1 treatment, implying that TRPM4 in vivo may be upregulated by fibrogenesis promotor TGFβ1. The upregulated TRPM4 in HF fibroblasts suggests that TRPM4 may play a role in cardiac fibrogenesis under various pathological conditions.

Pathological activation of CaMKII induces arrhythmogenicity through TRPM4 overactivation

TRPM4 is a Ca²⁺-activated nonselective cation channel involved in cardiovascular physiology and pathophysiology. Based on cellular experiments and numerical simulations, the present study aimed to explore the potential arrhythmogenicity of CaMKII-mediated TRPM4 channel overactivation linked to Ca²⁺ dysregulation in the heart. The confocal immunofluorescence microscopy, western blot, and proximity ligation assay (PLA) in HL-1 atrial cardiomyocytes and/or TRPM4-expressing TSA201 cells suggested that TRPM4 and CaMKII proteins are closely localized. Co-expression of TRPM4 and CaMKIIδ or a FRET-based sensor Camui in HEK293 cells showed that the extent of TRPM4 channel activation was correlated with that of CaMKII activity, suggesting their functional interaction. Both expressions and interaction of the two proteins were greatly enhanced by angiotensin II treatment, which induced early afterdepolarizations (EADs) at the repolarization phase of action potentials (APs) recorded from HL-1 cells by the current clamp mode of patch clamp technique. This arrhythmic change disappeared after treatment with the TRPM4 channel blocker 9-phenanthrol or CaMKII inhibitor KN-62. In order to quantitatively assess how CaMKII modulates the gating behavior of TRPM4 channel, the ionomycin-permeabilized cell-attached recording was employed to obtain the voltage-dependent parameters such as steady-state open probability and time constants for activation/deactivation at different [Ca²⁺]_i. Numerical simulations incorporating these kinetic data into a modified HL-1 model indicated that > 3-fold increase in TRPM4 current density induces EADs at the late repolarization phase and CaMKII inhibition (by KN-62) completely eliminates them. These results collectively suggest a novel arrhythmogenic mechanism involving excessive CaMKII activity that causes TRPM4 overactivation in the stressed heart.

Inherited Cardiac Arrhythmia Syndromes: Focus on Molecular Mechanisms Underlying TRPM4 Channelopathies

The Transient Receptor Potential Melastatin 4 (TRPM4) is a transmembrane N-glycosylated ion channel that belongs to the large family of TRP proteins. It has an equal permeability to Na⁺ and K⁺ and is activated via an increase of the intracellular calcium concentration and membrane depolarization. Due to its wide distribution, TRPM4 dysfunction has been linked with several pathophysiological processes, including inherited cardiac arrhythmias. Many pathogenic variants of the TRPM4 gene have been identified in patients with different forms of cardiac disorders such as conduction defects, Brugada syndrome, and congenital long QT syndrome. At the cellular level, these variants induce either gain- or loss-of-function of TRPM4 channels for similar clinical phenotypes. However, the molecular mechanisms associating these functional alterations to the clinical phenotypes remain poorly understood. The main objective of this article is to review the major cardiac TRPM4 channelopathies and recent advances regarding their genetic background and the underlying molecular mechanisms.

TRPM Channels in Human Diseases

The transient receptor potential melastatin (TRPM) subfamily belongs to the TRP cation channels family. Since the first cloning of TRPM1 in 1989, tremendous progress has been made in identifying novel members of the TRPM subfamily and their functions. The TRPM subfamily is composed of eight members consisting of four six-transmembrane domain subunits, resulting in homomeric or heteromeric channels. From a structural point of view, based on the homology sequence of the coiled-coil in the C-terminus, the eight TRPM members are clustered into four groups: TRPM1/M3, M2/M8, M4/M5 and M6/M7. TRPM subfamily members have been involved in several physiological functions. However, they are also linked to diverse pathophysiological human processes. Alterations in the expression and function of TRPM subfamily ion channels might generate several human diseases including cardiovascular and neurodegenerative alterations, organ dysfunction, cancer and many other channelopathies. These effects position them as remarkable putative targets for novel diagnostic strategies, drug design and therapeutic approaches. Here, we review the current knowledge about the main characteristics of all members of the TRPM family, focusing on their actions in human diseases.

New role of TRPM4 channel in the cardiac excitation-contraction coupling in response to physiological and pathological hypertrophy in mouse

The transient receptor potential Melastatin 4 (TRPM4) channel is a calcium-activated non-selective cation channel expressed widely. In the heart, using a knock-out mouse model, the TRPM4 channel has been shown to be involved in multiple processes, including β-adrenergic regulation, cardiac conduction, action potential duration and hypertrophic adaptations. This channel was recently shown to be involved in stress-induced cardiac arrhythmias in a mouse model overexpressing TRPM4 in ventricular cardiomyocytes. However, the link between TRPM4 channel expression in ventricular cardiomyocytes, the hypertrophic response to stress and/or cellular arrhythmias has yet to be elucidated. In this present study, we induced pathological hypertrophy in response to myocardial infarction using a mouse model of Trpm4 gene invalidation, and demonstrate that TRPM4 is essential for survival. We also demonstrate that the TRPM4 is required to activate both the Akt and Calcineurin pathways. Finally, using two hypertrophy models, either a physiological response to endurance training or a pathological response to myocardial infarction, we show that TRPM4 plays a role in regulating transient calcium amplitudes and leads to the development of cellular arrhythmias potentially in cooperation with the Sodium-calcium exchange (NCX). Here, we report two functions of the TRPM4 channel: first its role in adaptive hypertrophy, and second its association with NCX could mediate transient calcium amplitudes which trigger cellular arrhythmias.

Targeting Ca2 + Handling Proteins for the Treatment of Heart Failure and Arrhythmias

Diseases of the heart, such as heart failure and cardiac arrhythmias, are a growing socio-economic burden. Calcium (Ca²⁺) dysregulation is key hallmark of the failing myocardium and has long been touted as a potential therapeutic target in the treatment of a variety of cardiovascular diseases (CVD). In the heart, Ca²⁺ is essential for maintaining normal cardiac function through the generation of the cardiac action potential and its involvement in excitation contraction coupling. As such, the proteins which regulate Ca²⁺ cycling and signaling play a vital role in maintaining Ca²⁺ homeostasis. Changes to the expression levels and function of Ca²⁺-channels, pumps and associated intracellular handling proteins contribute to altered Ca²⁺ homeostasis in CVD. The remodeling of Ca²⁺-handling proteins therefore results in impaired Ca²⁺ cycling, Ca²⁺ leak from the sarcoplasmic reticulum and reduced Ca²⁺ clearance, all of which contributes to increased intracellular Ca²⁺. Currently, approved treatments for targeting Ca²⁺ handling dysfunction in CVD are focused on Ca²⁺ channel blockers. However, whilst Ca²⁺ channel blockers have been successful in the treatment of some arrhythmic disorders, they are not universally prescribed to heart failure patients owing to their ability to depress cardiac function. Despite the progress in CVD treatments, there remains a clear need for novel therapeutic approaches which are able to reverse pathophysiology associated with heart failure and arrhythmias. Given that heart failure and cardiac arrhythmias are closely associated with altered Ca²⁺ homeostasis, this review will address the molecular changes to proteins associated with both Ca²⁺-handling and -signaling; their potential as novel therapeutic targets will be discussed in the context of pre-clinical and, where available, clinical data.

TRPM4 Modulates Right Ventricular Remodeling Under Pressure Load Accompanied With Decreased Expression Level

BACKGROUND

Survival of patients with congenital heart defects including increased right ventricular pressure load (ie, tetralogy of Fallot) or pulmonary hypertension is dependent on the function of the right ventricle (RV). RV remodeling has several effects with progressive transition from compensated status to heart failure. Transient receptor potential melastatin 4 (TRPM4) forms cation channels expressed in myocardium, which was shown to modulate cardiac remodeling in the left ventricle of mice. Aim of this study was to identify the role of TRPM4 for contractile function and remodeling of the RV in a rat model of right ventricular pressure load.

METHODS AND RESULTS

We performed experiments with untreated rats and under monocrotaline (MCT)-induced pressure load comparing wild-type (Trpm4^+/+) and TRPM4-deficient (Trpm4^-/-) rats. RV function was characterized by echocardiography and contractility measurements of isolated papillary muscles. RV hypertrophy was investigated by echocardiography and by determination of hypertrophy indices. Pulmonary arterial remodeling was evaluated by echocardiography and histology. TRPM4 protein expression in RV of human, rat and mouse was detected by Western blot and quantified in rat. TRPM4 proteins were detected in RV myocardium of rat and mouse, which were not detectable in TRPM4-deficient animals. Proteins of the same size were found in RV of a pediatric patient with tetralogy of Fallot. In untreated status, Trpm4^+/+ and Trpm4^-/- rats showed comparable RV contractile function and dimensions. Under pressure load (42 days after MCT injection), RV hypertrophy was significantly increased in Trpm4^-/- rats compared with Trpm4^+/+ controls, whereas MCT-mediated alterations in cardiac contractility and pulmonary arterial remodeling were not affected by TRPM4 inactivation in rats. Finally, TRPM4 protein expression in RV was drastically reduced in MCT-treated rats, whereas left ventricle of the same animals showed no alteration in TRPM4 expression.

CONCLUSIONS

Right ventricular pressure load evoked by MCT treatment in rats leads to a prominent downregulation of TRPM4 protein expression in the RV and complete deletion of TRPM4 expression aggravates right ventricular hypertrophy. Thus, therapeutic modulation of TRPM4 expression and activity might represent a novel approach to target right ventricular remodeling in patients with pulmonary hypertension or otherwise loaded RV.

TRPM4 channel inhibitors 9-phenanthrol and glibenclamide differentially decrease guinea pig detrusor smooth muscle whole-cell cation currents and phasic contractions

Nonselective cation channels, consistent with transient receptor potential melastatin-4 (TRPM4), regulate detrusor smooth muscle (DSM) function. TRPM4 channels can exist as homomers or assemble with sulfonylurea receptors (SURs) as complexes. We evaluated contributions of TRPM4/SUR-TRPM4 channels to DSM excitability and contractility by examining the effects of TRPM4/SUR-TRPM4 channel modulators 9-phenanthrol, glibenclamide, and diazoxide on freshly-isolated guinea pig DSM cells (amphotericin-B perforated patch-clamp electrophysiology) and mucosa-free DSM strips (isometric tension recordings). In DSM cells, complete removal of extracellular Na⁺ decreased voltage-step-induced cation (non-K⁺ selective) currents. At high positive membrane potentials, 9-phenanthrol at 100 μM attenuated voltage step-induced currents more effectively than at 30 μM, revealing concentration-dependent, voltage-sensitive inhibition. In comparison to 9-phenanthrol, glibenclamide (100 μM) displayed lower inhibition of cation currents. In the presence of glibenclamide (100 μM), 9-phenanthrol (100 μM) further decreased the currents. The SUR-TRPM4 complex activator diazoxide (100-300 μM) weakly inhibited the currents. 9-Phenanthrol, but not glibenclamide or diazoxide, increased cell capacitance (a cell surface area indicator). In contractility studies, glibenclamide displayed lower potencies than 9-phenanthrol attenuating spontaneous and 20 mM KCl-induced DSM phasic contractions. While both compounds showed similar maximum inhibitions on DSM spontaneous phasic contractions, glibenclamide was generally less efficacious on 20 mM KCl-induced phasic contractions. In summary, the observed differential effects of 9-phenanthrol and glibenclamide on DSM excitability and contractility support unique mechanisms for the two compounds. The data suggest that SUR-TRPM4 complexes do not contribute to DSM function. This study advances our understanding of pharmacological effects of glibenclamide and 9-phenanthrol on DSM cell cation currents.

Transient receptor potential channels in cardiac health and disease

Transient receptor potential (TRP) channels are nonselective cationic channels that are generally Ca²⁺ permeable and have a heterogeneous expression in the heart. In the myocardium, TRP channels participate in several physiological functions, such as modulation of action potential waveform, pacemaking, conduction, inotropy, lusitropy, Ca²⁺ and Mg²⁺ handling, store-operated Ca²⁺ entry, embryonic development, mitochondrial function and adaptive remodelling. Moreover, TRP channels are also involved in various pathological mechanisms, such as arrhythmias, ischaemia-reperfusion injuries, Ca²⁺-handling defects, fibrosis, maladaptive remodelling, inherited cardiopathies and cell death. In this Review, we present the current knowledge of the roles of TRP channels in different cardiac regions (sinus node, atria, ventricles and Purkinje fibres) and cells types (cardiomyocytes and fibroblasts) and discuss their contribution to pathophysiological mechanisms, which will help to identify the best candidates for new therapeutic targets among the cardiac TRP family.

Time Course of Low-Frequency Oscillatory Behavior in Human Ventricular Repolarization Following Enhanced Sympathetic Activity and Relation to Arrhythmogenesis

Background and Objectives: Recent studies in humans and dogs have shown that ventricular repolarization exhibits a low-frequency (LF) oscillatory pattern following enhanced sympathetic activity, which has been related to arrhythmic risk. The appearance of LF oscillations in ventricular repolarization is, however, not immediate, but it may take up to some minutes. This study seeks to characterize the time course of the action potential (AP) duration (APD) oscillatory behavior in response to sympathetic provocations, unveil its underlying mechanisms and establish a potential link to arrhythmogenesis under disease conditions.

Materials and Methods: A representative set of human ventricular computational models coupling cellular electrophysiology, calcium dynamics, β-adrenergic signaling, and mechanics was built. Sympathetic provocation was modeled via phasic changes in β-adrenergic stimulation (β-AS) and mechanical stretch at Mayer wave frequencies within the 0.03–0.15 Hz band.

Results: Our results show that there are large inter-individual differences in the time lapse for the development of LF oscillations in APD following sympathetic provocation, with some cells requiring just a few seconds and other cells needing more than 3 min. Whereas, the oscillatory response to phasic mechanical stretch is almost immediate, the response to β-AS is much more prolonged, in line with experimentally reported evidences, thus being this component the one driving the slow development of APD oscillations following enhanced sympathetic activity. If β-adrenoceptors are priorly stimulated, the time for APD oscillations to become apparent is remarkably reduced, with the oscillation time lapse being an exponential function of the pre-stimulation level. The major mechanism underlying the delay in APD oscillations appearance is related to the slow I_Ks phosphorylation kinetics, with its relevance being modulated by the I_Ks conductance of each individual cell. Cells presenting short oscillation time lapses are commonly associated with large APD oscillation magnitudes, which facilitate the occurrence of pro-arrhythmic events under disease conditions involving calcium overload and reduced repolarization reserve.

Conclusions: The time course of LF oscillatory behavior of APD in response to increased sympathetic activity presents high inter-individual variability, which is associated with different expression and PKA phosphorylation kinetics of the I_Ks current. Short time lapses in the development of APD oscillations are associated with large oscillatory magnitudes and pro-arrhythmic risk under disease conditions.

TRP Channels Expression Profile in Human End-Stage Heart Failure

Objectives: Many studies indicate the involvement of transient receptor potential (TRP) channels in the development of heart hypertrophy. However, the data is often conflicted and has originated in animal models. Here, we provide systematic analysis of TRP channels expression in human failing myocardium. Methods and results: Left-ventricular tissue samples were isolated from explanted hearts of NYHA III-IV patients undergoing heart transplants (n = 43). Quantitative real-time PCR was performed to assess the mRNA levels of TRPC, TRPM and TRPV channels. Analysis of functional, clinical and biochemical data was used to confirm an end-stage heart failure diagnosis. Compared to myocardium samples from healthy donor hearts (n = 5), we detected a distinct increase in the expression of TRPC1, TRPC5, TRPM4 and TRPM7, and decreased expression of TRPC4 and TRPV2. These changes were not dependent on gender, clinical or biochemical parameters, nor functional parameters of the heart. We detected, however, a significant correlation of TRPC1 and MEF2c expression. Conclusions: The end-stage heart failure displays distinct expressional changes of TRP channels. Our findings provide a systematic description of TRP channel expression in human heart failure. The results highlight the complex interplay between TRP channels and the need for deeper analysis of early stages of hypertrophy and heart failure development.

Atria-selective antiarrhythmic drugs in need of alliance partners

Atria-selective antiarrhythmic drugs in need of alliance partners. Guideline-based treatment of atrial fibrillation (AF) comprises prevention of thromboembolism and stroke, as well as antiarrhythmic therapy by drugs, electrical rhythm conversion, ablation and surgical procedures. Conventional antiarrhythmic drugs are burdened with unwanted side effects including a propensity of triggering life-threatening ventricular fibrillation. In order to solve this therapeutic dilemma, 'atria-selective' antiarrhythmic drugs have been developed for the treatment of supraventricular arrhythmias. These drugs are designed to aim at atrial targets, taking advantage of differences in atrial and ventricular ion channel expression and function. However it is not clear, whether such drugs are sufficiently antiarrhythmic or whether they are in need of an alliance partner for clinical efficacy. Atria-selective Na⁺ channel blockers display fast dissociation kinetics and high binding affinity to inactivated channels. Compounds targeting atria-selective K⁺ channels include blockers of ultra rapid delayed rectifier (Kv1.5) or acetylcholine-activated inward rectifier K⁺ channels (Kir3.x), inward rectifying K⁺ channels (Kir2.x), Ca²⁺-activated K⁺ channels of small conductance (SK), weakly rectifying two-pore domain K⁺ channels (K_2P), and transient receptor potential channels (TRP). Despite good antiarrhythmic data from in-vitro and animal model experiments, clinical efficacy of atria-selective antiarrhythmic drugs remains to be demonstrated. In the present review we will briefly summarize the novel compounds and their proposed antiarrhythmic action. In addition, we will discuss the evidence for putative improvement of antiarrhythmic efficacy and potency by addressing multiple pathophysiologically relevant targets as possible alliance partners.

TRP Channels: Current Perspectives in the Adverse Cardiac Remodeling

Calcium is an important second messenger required not only for the excitation-contraction coupling of the heart but also critical for the activation of cell signaling pathways involved in the adverse cardiac remodeling and consequently for the heart failure. Sustained neurohumoral activation, pressure-overload, or myocardial injury can cause pathologic hypertrophic growth of the heart followed by interstitial fibrosis. The consequent heart’s structural and molecular adaptation might elevate the risk of developing heart failure and malignant arrhythmia. Compelling evidences have demonstrated that Ca²⁺ entry through TRP channels might play pivotal roles in cardiac function and pathology. TRP proteins are classified into six subfamilies: TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPA (ankyrin), TRPML (mucolipin), and TRPP (polycystin), which are activated by numerous physical and/or chemical stimuli. TRP channels participate to the handling of the intracellular Ca²⁺ concentration in cardiac myocytes and are mediators of different cardiovascular alterations. This review provides an overview of the current knowledge of TRP proteins implication in the pathologic process of some frequent cardiac diseases associated with the adverse cardiac remodeling such as cardiac hypertrophy, fibrosis, and conduction alteration.

Role of the TRPM4 Channel in Cardiovascular Physiology and Pathophysiology

The transient receptor potential cation channel subfamily M member 4 (TRPM4) channel influences calcium homeostasis during many physiological activities such as insulin secretion, immune response, respiratory reaction, and cerebral vasoconstriction. This calcium-activated, monovalent, selective cation channel also plays a key role in cardiovascular pathophysiology; for example, a mutation in the TRPM4 channel leads to cardiac conduction disease. Recently, it has been suggested that the TRPM4 channel is also involved in the development of cardiac ischemia-reperfusion injury, which causes myocardial infarction. In the present review, we discuss the physiological function of the TRPM4 channel, and assess its role in cardiovascular pathophysiology.

Transient receptor potential melastatin 4 channel inhibitor 9-phenanthrol inhibits K+ but not Ca2+ currents in canine ventricular myocytes

The role of transient receptor potential melastatin 4 (TRPM4) channels has been frequently tested using their inhibitor 9-phenanthrol in various cardiac preparations; however, the selectivity of the compound is uncertain. Therefore, in the present study, the concentration-dependent effects of 9-phenanthrol on major ionic currents were studied in canine isolated ventricular cells using whole-cell configuration of the patch-clamp technique and 10 mM BAPTA-containing pipette solution to prevent the Ca²⁺-dependent activation of TRPM4 channels. Transient outward (I_to1), rapid delayed rectifier (I_Kr), and inward rectifier (I_K1) K⁺ currents were suppressed by 10 and 30 μM 9-phenanthrol with the blocking potency for I_K1 < I_Kr < I_to1 and partial reversibility. L-type Ca²⁺ current was not affected up to the concentration of 30 μM. In addition, a steady outward current was detected at voltages positive to -40 mV in 9-phenanthrol, which was larger at more positive voltages and larger 9-phenanthrol concentrations. Action potentials were recorded using microelectrodes. Maximal rate of depolarization, phase-1 repolarization, and terminal repolarization were decreased and the plateau potential was depressed by 9-phenanthrol (3-30 μM), congruently with the observed alterations of ionic currents. Significant action potential prolongation was observed by 9-phenanthrol in the majority of the studied cells, but only at 30 μM concentration. In conclusion, 9-phenanthrol is not selective to TRPM4 channels in canine ventricular myocardium; therefore, its application as a TRPM4 blocker can be appropriate only in expression systems but not in native cardiac cells.

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.

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 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.

Enhanced parenchymal arteriole tone and astrocyte signaling protect neurovascular coupling mediated parenchymal arteriole vasodilation in the spontaneously hypertensive rat

Functional hyperemia is the regional increase in cerebral blood flow upon increases in neuronal activity which ensures that the metabolic demands of the neurons are met. Hypertension is known to impair the hyperemic response; however, the neurovascular coupling mechanisms by which this cerebrovascular dysfunction occurs have yet to be fully elucidated. To determine whether altered cortical parenchymal arteriole function or astrocyte signaling contribute to blunted neurovascular coupling in hypertension, we measured parenchymal arteriole reactivity and vascular smooth muscle cell Ca(2+) dynamics in cortical brain slices from normotensive Wistar Kyoto (WKY) and spontaneously hypertensive (SHR) rats. We found that vasoconstriction in response to the thromboxane A2 receptor agonist U46619 and basal vascular smooth muscle cell Ca(2+) oscillation frequency were significantly increased in parenchymal arterioles from SHR. In perfused and pressurized parenchymal arterioles, myogenic tone was significantly increased in SHR. Although K(+)-induced parenchymal arteriole dilations were similar in WKY and SHR, metabotropic glutamate receptor activation-induced parenchymal arteriole dilations were enhanced in SHR. Further, neuronal stimulation-evoked parenchymal arteriole dilations were similar in SHR and WKY. Our data indicate that neurovascular coupling is not impaired in SHR, at least at the level of the parenchymal arterioles.

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.

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.

Molecular candidates for cardiac stretch-activated ion channels

The heart is a mechanically-active organ that dynamically senses its own mechanical environment. This environment is constantly changing, on a beat-by-beat basis, with additional modulation by respiratory activity and changes in posture or physical activity, and further overlaid with more slowly occurring physiological (e.g. pregnancy, endurance training) or pathological challenges (e.g. pressure or volume overload). Far from being a simple pump, the heart detects changes in mechanical demand and adjusts its performance accordingly, both via heart rate and stroke volume alteration. Many of the underlying regulatory processes are encoded intracardially, and are thus maintained even in heart transplant recipients. Over the last three decades, molecular substrates of cardiac mechanosensitivity have gained increasing recognition in the scientific and clinical communities. Nonetheless, the processes underlying this phenomenon are still poorly understood. Stretch-activated ion channels (SAC) have been identified as one contributor to mechanosensitive autoregulation of the heartbeat. They also appear to play important roles in the development of cardiac pathologies – most notably stretch-induced arrhythmias. As recently discovered, some established cardiac drugs act, in part at least, via mechanotransduction pathways suggesting SAC as potential therapeutic targets. Clearly, identification of the molecular substrate of cardiac SAC is of clinical importance and a number of candidate proteins have been identified. At the same time, experimental studies have revealed variable–and at times contrasting–results regarding their function. Further complication arises from the fact that many ion channels that are not classically defined as SAC, including voltage and ligand-gated ion channels, can respond to mechanical stimulation. Here, we summarise what is known about the molecular substrate of the main candidates for cardiac SAC, before identifying potential further developments in this area of translational research.

The TRPM4 channel inhibitor 9-phenanthrol

The phenanthrene-derivative 9-phenanthrol is a recently identified inhibitor of the transient receptor potential melastatin (TRPM) 4 channel, a Ca(2+) -activated non-selective cation channel whose mechanism of action remains to be determined. Subsequent studies performed on other ion channels confirm the specificity of the drug for TRPM4. In addition, 9-phenanthrol modulates a variety of physiological processes through TRPM4 current inhibition and thus exerts beneficial effects in several pathological conditions. 9-Phenanthrol modulates smooth muscle contraction in bladder and cerebral arteries, affects spontaneous activity in neurons and in the heart, and reduces lipopolysaccharide-induced cell death. Among promising potential applications, 9-phenanthrol exerts cardioprotective effects against ischaemia-reperfusion injuries and reduces ischaemic stroke injuries. In addition to reviewing the biophysical effects of 9-phenanthrol, here we present information about its appropriate use in physiological studies and possible clinical applications.

Glycosylation of TRPM4 and TRPM5 channels: molecular determinants and functional aspects

The transient receptor potential channel, TRPM4, and its closest homolog, TRPM5, are non-selective cation channels that are activated by an increase in intracellular calcium. They are expressed in many cell types, including neurons and myocytes. Although the electrophysiological and pharmacological properties of these two channels have been previously studied, less is known about their regulation, in particular their post-translational modifications. We, and others, have reported that wild-type (WT) TRPM4 channels expressed in HEK293 cells, migrated on SDS-PAGE gel as doublets, similar to other ion channels and membrane proteins. In the present study, we provide evidence that TRPM4 and TRPM5 are each N-linked glycosylated at a unique residue, Asn⁹⁹² and Asn⁹³², respectively. N-linked glycosylated TRPM4 is also found in native cardiac cells. Biochemical experiments using HEK293 cells over-expressing WT TRPM4/5 or N992Q/N932Q mutants demonstrated that the abolishment of N-linked glycosylation did not alter the number of channels at the plasma membrane. In parallel, electrophysiological experiments demonstrated a decrease in the current density of both mutant channels, as compared to their respective controls, either due to the Asn to Gln mutations themselves or abolition of glycosylation. To discriminate between these possibilities, HEK293 cells expressing TRPM4 WT were treated with tunicamycin, an inhibitor of glycosylation. In contrast to N-glycosylation signal abolishment by mutagenesis, tunicamycin treatment led to an increase in the TRPM4-mediated current. Altogether, these results demonstrate that TRPM4 and TRPM5 are both N-linked glycosylated at a unique site and also suggest that TRPM4/5 glycosylation seems not to be involved in channel trafficking, but mainly in their functional regulation.

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.