Age-dependent decrease in TRPM4 channel expression but not trafficking alters urinary bladder smooth muscle contractility

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.

Differential effects of TRPM4 channel inhibitors on Guinea pig urinary bladder smooth muscle excitability and contractility: Novel 4-chloro-2-[2-(2-chloro-phenoxy)-acetylamino]-benzoic acid (CBA) versus classical 9-phenanthrol

Non-selective cation channels in urinary bladder smooth muscle (UBSM) are thought to mediate increases in cellular excitability and contractility. For transient receptor potential melastatin type-4 (TRPM4) channels, the evidence primarily relies on the inhibitor 9-phenanthrol, which exhibits pharmacological limitations. Recently, 4-chloro-2-[2-(2-chloro-phenoxy)-acetylamino]-benzoic acid (CBA) has been discovered as a novel TRPM4 channel blocker. We examined how, in comparison to 9-phenanthrol, CBA affects the excitability of freshly isolated guinea pig UBSM cells and the contractility of UBSM strips. Additionally, non-selective TRPM4 channel inhibitor flufenamic acid (FFA) and potentiator BTP2 (also known as YM-58483) were studied in UBSM cells. Unlike robust inhibition for 9-phenanthrol already known, CBA (up to 100 μM) displayed either no or a very weak reduction (<20%) in spontaneous phasic, 20 mM KCl-induced, and electrical field stimulated contractions. For 300 μM CBA, reductions were higher except for an increase in the frequency of KCl-induced contractions. In UBSM cells, examined under amphotericin B-perforated patch-clamp, CBA (30 μM) did not affect the membrane potential (I = 0) or voltage step-induced whole-cell cation currents, sensitive to 9-phenanthrol. The currents were not inhibited by FFA (100 μM), increased by BTP2 (10 μM), nor enhanced under a strongly depolarizing holding voltage of -16 or + 6 mV (vs. -74 mV). None of the three compounds affected the cell capacitance, unlike 9-phenanthrol. In summary, the novel inhibitor CBA and nonselective FFA did not mimic the inhibitory properties of 9-phenanthrol on UBSM function. These results suggest that TRPM4 channels, although expressed in UBSM, play a distinct role rather than direct regulation of excitability and contractility.

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.