The non-selective monovalent cationic channels TRPM4 and TRPM5

Design, synthesis, and biological evaluation of 2-(naphthalen-1-yloxy)-N-phenylacetamide derivatives as TRPM4 inhibitors for the treatment of prostate cancer

Transient receptor potential melastatin 4 (TRPM4) is considered to be a potential target for cancer and other human diseases. Herein, a series of 2-(naphthalen-1-yloxy)-N-phenylacetamide derivatives were designed and synthesized as new TRPM4 inhibitors, aiming to improve cellular potency. One of the most promising compounds, 7d (ZX08903), displayed promising antiproliferative activity against prostate cancer cell lines. 7d also suppressed colony formation and the expression of androgen receptor (AR) protein in prostate cancer cells. Furthermore, 7d can concentration-dependently induce cell apoptosis in prostate cancer cells. Collectively, these findings indicated that compound 7d may serve as a promising lead compound for further anticancer drug development.

Mechanosensing in Metabolism

Electrical mechanosensing is a process mediated by specialized ion channels, gated directly or indirectly by mechanical forces, which allows cells to detect and subsequently respond to mechanical stimuli. The activation of mechanosensitive (MS) ion channels, intrinsically gated by mechanical forces, or mechanoresponsive (MR) ion channels, indirectly gated by mechanical forces, results in electrical signaling across lipid bilayers, such as the plasma membrane. While the functions of mechanically gated channels within a sensory context (e.g., proprioception and touch) are well described, there is emerging data demonstrating functions beyond touch and proprioception, including mechanoregulation of intracellular signaling and cellular/systemic metabolism. Both MR and MS ion channel signaling have been shown to contribute to the regulation of metabolic dysfunction, including obesity, insulin resistance, impaired insulin secretion, and inflammation. This review summarizes our current understanding of the contributions of several MS/MR ion channels in cell types implicated in metabolic dysfunction, namely, adipocytes, pancreatic β-cells, hepatocytes, and skeletal muscle cells, and discusses MS/MR ion channels as possible therapeutic targets. © 2024 American Physiological Society. Compr Physiol 14:5269-5290, 2024.

TRP (transient receptor potential) ion channel family: structures, biological functions and therapeutic interventions for diseases

Transient receptor potential (TRP) channels are sensors for a variety of cellular and environmental signals. Mammals express a total of 28 different TRP channel proteins, which can be divided into seven subfamilies based on amino acid sequence homology: TRPA (Ankyrin), TRPC (Canonical), TRPM (Melastatin), TRPML (Mucolipin), TRPN (NO-mechano-potential, NOMP), TRPP (Polycystin), TRPV (Vanilloid). They are a class of ion channels found in numerous tissues and cell types and are permeable to a wide range of cations such as Ca²⁺, Mg²⁺, Na⁺, K⁺, and others. TRP channels are responsible for various sensory responses including heat, cold, pain, stress, vision and taste and can be activated by a number of stimuli. Their predominantly location on the cell surface, their interaction with numerous physiological signaling pathways, and the unique crystal structure of TRP channels make TRPs attractive drug targets and implicate them in the treatment of a wide range of diseases. Here, we review the history of TRP channel discovery, summarize the structures and functions of the TRP ion channel family, and highlight the current understanding of the role of TRP channels in the pathogenesis of human disease. Most importantly, we describe TRP channel-related drug discovery, therapeutic interventions for diseases and the limitations of targeting TRP channels in potential clinical applications.

The TRPM4 channel inhibitor 9-phenanthrol alleviates cerebral edema after traumatic brain injury in rats

Cerebral edema (CE) exerts an important effect on brain injury after traumatic brain injury (TBI). Upregulation of transient receptor potential melastatin 4 (TRPM4) in vascular endothelial cells (ECs) results in damage to capillaries and the blood-brain barrier (BBB), which is critical for the development of CE. Many studies have shown that 9-phenanthrol (9-PH) effectively inhibits TRPM4. The current study aimed to investigate the effect of 9-PH on reducing CE after TBI. In this experiment, we observed that 9-PH markedly reduced brain water content, BBB disruption, proliferation of microglia and astrocytes, neutrophil infiltration, neuronal apoptosis and neurobehavioral deficits. At the molecular level, 9-PH significantly inhibited the protein expression of TRPM4 and MMP-9, alleviated the expression of apoptosis-related molecules and inflammatory cytokines, such as Bax, TNF-α and IL-6, near injured tissue, and diminished serum SUR1 and TRPM4 levels. Mechanistically, treatment with 9-PH inhibited activation of the PI3K/AKT/NF-kB signaling pathway, which was reported to be involved in the expression of MMP-9. Taken together, the results of this study indicate that 9-PH effectively reduces CE and alleviates secondary brain injury partly through the following possible mechanisms: ①9-PH inhibits TRPM4-mediated Na + influx and reduces cytotoxic CE; ②9-PH hinders the expression and activity of MMP-9 by inhibiting the TRPM4 channel and decreases disruption of the BBB, thereby preventing vasogenic cerebral edema. ③9-PH reduces further inflammatory and apoptotic damage to tissues.

On the modulation of TRPM channels: Current perspectives and anticancer therapeutic implications

The transient melastatin receptor potential (TRPM) ion channel subfamily functions as cellular sensors and transducers of critical biological signal pathways by regulating ion homeostasis. Some members of TRPM have been cloned from cancerous tissues, and their abnormal expressions in various solid malignancies have been correlated with cancer cell growth, survival, or death. Recent evidence also highlights the mechanisms underlying the role of TRPMs in tumor epithelial-mesenchymal transition (EMT), autophagy, and cancer metabolic reprogramming. These implications support TRPM channels as potential molecular targets and their modulation as an innovative therapeutic approach against cancer. Here, we discuss the general characteristics of the different TRPMs, focusing on current knowledge about the connection between TRPM channels and critical features of cancer. We also cover TRPM modulators used as pharmaceutical tools in biological trials and an indication of the only clinical trial with a TRPM modulator about cancer. To conclude, the authors describe the prospects for TRPM channels in oncology.

Involvement of TRPM7 in Alcohol-Induced Damage of the Blood-Brain Barrier in the Presence of HIV Viral Proteins

Ethanol (EtOH) exerts its effects through various protein targets, including transient receptor potential melastatin 7 (TRPM7) channels, which play an essential role in cellular homeostasis. We demonstrated that TRPM7 is expressed in rat brain microvascular endothelial cells (rBMVECs), the major cellular component of the blood-brain barrier (BBB). Heavy alcohol drinking is often associated with HIV infection, however mechanisms underlying alcohol-induced BBB damage and HIV proteins, are not fully understood. We utilized the HIV-1 transgenic (HIV-1Tg) rat to mimic HIV-1 patients on combination anti-retroviral therapy (cART) and demonstrated TRPM7 expression in rBMVECs wass lower in adolescent HIV-1Tg rats compared to control animals, however control and HIV-1Tg rats expressed similar levels at 9 weeks, indicating persistent presence of HIV-1 proteins delayed TRPM7 expression. Binge exposure to EtOH (binge EtOH) decreased TRPM7 expression in control rBMVECs in a concentration-dependent manner, and abolished TRPM7 expression in HIV-1Tg rats. In human BMVECs (hBMVECs), TRPM7 expression was downregulated after treatment with EtOH, HIV-1 proteins, and in combination. Next, we constructed in vitro BBB models using BMVECs and found TRPM7 antagonists enhanced EtOH-mediated BBB integrity changes. Our study demonstrated alcohol decreased TRPM7 expression, whereby TRPM7 could be involved in the mechanisms underlying BBB alcohol-induced damage in HIV-1 patients on cART.

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.

A glibenclamide-sensitive TRPM4-mediated component of CA1 excitatory postsynaptic potentials appears in experimental autoimmune encephalomyelitis

The transient receptor potential melastatin 4 (TRPM4) channel contributes to disease severity in the murine experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis and to neuronal cell death in models of excitotoxicity and traumatic brain injury. As TRPM4 is activated by intracellular calcium and conducts monovalent cations, we hypothesized that TRPM4 may contribute to and boost excitatory synaptic transmission in CA1 pyramidal neurons of the hippocampus. Using single-spine calcium imaging and electrophysiology, we found no effect of the TRPM4 antagonists 9-phenanthrol and glibenclamide on synaptic transmission in hippocampal slices from healthy mice. In contrast, glibenclamide but not 9-phenanthrol reduced excitatory synaptic potentials in slices from EAE mice, an effect that was absent in slices from EAE mice lacking TRPM4. We conclude that TRPM4 plays little role in basal hippocampal synaptic transmission, but a glibenclamide-sensitive TRPM4-mediated contribution to excitatory postsynaptic responses is upregulated at the acute phase of EAE.

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.

Thermodynamic and structural basis of temperature-dependent gating in TRP channels

Living organisms require detecting the environmental thermal clues for survival, allowing them to avoid noxious stimuli or find prey moving in the dark. In mammals, the Transient Receptor Potential ion channels superfamily is constituted by 27 polymodal receptors whose activity is controlled by small ligands, peptide toxins, protons and voltage. The thermoTRP channels subgroup exhibits unparalleled temperature dependence -behaving as heat and cold sensors. Functional studies have dissected their biophysical features in detail, and the advances of single-particle Cryogenic Electron microscopy provided the structural framework required to propose detailed channel gating mechanisms. However, merging structural and functional evidence for temperature-driven gating of thermoTRP channels has been a hard nut to crack, remaining an open question nowadays. Here we revisit the highlights on the study of heat and cold sensing in thermoTRP channels in the light of the structural data that has emerged during recent years.

Expression of transient receptor potential melastatin 4 in differential diagnosis of eosinophilic renal tumors

Immunohistochemical and molecular studies to differentiate eosinophilic kidney tumors are gradually increasing. The present study investigated the role of transient receptor potential cation channel subfamily M member 4 (TRPM4), a non-selective cation channel associated with migration, proliferation and invasion in cancer cells, in this differentiation. The aim was to investigate the effectiveness of TRPM4 in differentiation of eosinophilic kidney tumors. The study included a total of 112 patients, including 97 eosinophilic kidney tumors with the diagnoses of 33 eosinophilic clear cell renal cell carcinoma (CCRCC), 35 eosinophilic chromophobe renal cell carcinoma (ChRCC), 8 papillary renal cell carcinoma type 2 (P2RCC), 21 renal oncocytoma (RO), as well as 15 papillary renal cell carcinoma type 1 to differentiate from P2RCC. For TRPM4, diffuse staining (>10%) was observed in 2 CCRCC, 15 ChRCC, 20 RO and 4 P2RCC cases. There was a significant difference between eosinophilic CCRCC and other eosinophilic tumors (P<0.05). While basolateral staining was observed in papillary tumors, membrane staining was observed in other stained cases. It was hypothesized that the use of TRPM4 along with morphological findings, cytokeratin 7 and other markers may be useful for the differentiation of eosinophilic kidney tumors.

Pivotal role of Transient Receptor Potential Channels in oral physiology

BACKGROUND

Transient Receptor Potential (TRP) Channels constitute a large family of non-selective permeable ion channels involved in the perception of environmental stimuli with a central and continuously expanding role in oral tissue homeostasis. Recent studies indicate the regulatory role of TRPs in pulp physiology, oral mucosa sensation, dental pain nociception and salivary gland secretion. This review provides an update on the diverse functions of TRP channels in the physiology of oral cavity, with emphasis on their cellular location, the underlying molecular mechanisms and clinical significance.

METHODS

A structured search of bibliographic databases (PubMed and MEDLINE) was performed for peer reviewed studies on TRP channels function on oral cavity physiology the last ten years. A qualitative content analysis was performed in screened papers and a critical discussion of main findings is provided.

RESULTS

TRPs expression has been detected in major cell types of the oral cavity, including odontoblasts, periodontal ligament, oral epithelial, salivary gland cells, and chondrocytes of temporomandibular joints, where they mediate signal perception and transduction of mechanical, thermal, and osmotic stimuli. They contribute to pulp physiology through dentin formation, mineralization, and periodontal ligament formation along with alveolar bone remodeling in dental pulp and periodontal ligament cells. TRPs are also involved in oral mucosa sensation, dental pain nociception, saliva secretion, swallowing reflex and temporomandibular joints' development.

CONCLUSION

Various TRP channels regulate oral cavity homeostasis, playing an important role in the transduction of external stimuli to intracellular signals in a cell type-specific manner and presenting promising drug targets for the development of pharmacological strategies to manage oral diseases.

Transient receptor potential vanilloid 4 channel participates in mouse ventricular electrical activity

The TRPV4 channel is a calcium-permeable channel (PCa/PNa ∼ 10). Its expression has been reported in ventricular myocytes, where it is involved in several cardiac pathological mechanisms. In this study, we investigated the implication of TRPV4 in ventricular electrical activity. Left ventricular myocytes were isolated from trpv4+/+ and trpv4-/- mice. TRPV4 membrane expression and its colocalization with L-type calcium channels (Cav1.2) was confirmed using Western blot biotinylation, immunoprecipitation, and immunostaining experiments. Then, electrocardiograms (ECGs) and patch-clamp recordings showed shortened QTc and action potential (AP) duration in trpv4-/- compared with trpv4+/+ mice. Thus, TRPV4 activator GSK1016790A produced a transient and dose-dependent increase in AP duration at 90% of repolarization (APD90) in trpv4+/+ but not in trpv4-/- myocytes or when combined with TRPV4 inhibitor GSK2193874 (100 nM). Hence, GSK1016790A increased calcium transient (CaT) amplitude in trpv4+/+ but not in trpv4-/- myocytes, suggesting that TRPV4 carries an inward Ca2+ current in myocytes. Conversely, TRPV4 inhibitor GSK2193874 (100 nM) alone reduced APD90 in trpv4+/+ but not in trpv4-/- myocytes, suggesting that TRPV4 prolongs AP duration in basal condition. Finally, introducing TRPV4 parameters in a mathematical model predicted the development of an inward TRPV4 current during repolarization that increases AP duration and CaT amplitude, in accord with what was found experimentally. This study shows for the first time that TRPV4 modulates AP and QTc durations. It would be interesting to evaluate whether TRPV4 could be involved in long QT-mediated ventricular arrhythmias.NEW & NOTEWORTHY Transient receptor potential vanilloid 4 (TRPV4) is expressed at the membrane of mouse ventricular myocytes and colocalizes with non-T-tubular L-type calcium channels. Deletion of trpv4 gene in mice results in shortened QT interval on electrocardiogram and reduced action potential duration of ventricular myocytes. Pharmacological activation of TRPV4 channel leads to increased action potential duration and increased calcium transient amplitude in trpv4-/- but not in trpv4-/- ventricular myocytes. To the contrary, TRPV4 channel pharmacological inhibition reduces action potential duration in trpv4+/+ but not in trpv4-/- myocytes. Integration of TRPV4 channel in a computational model of mouse action potential shows that the channel carries an inward current contributing to slowing down action potential repolarization and to increase calcium transient amplitude, similarly to what is observed experimentally. This study highlights for the first time the involvement of TRPV4 channel in ventricular electrical activity.

TRPM4 in Cancer-A New Potential Drug Target

Transient receptor potential melastatin 4 (TRPM4) is widely expressed in various organs and associated with cardiovascular and immune diseases. Lately, the interest in studies on TRPM4 in cancer has increased. Thus far, TRPM4 has been investigated in diffuse large B-cell lymphoma, prostate, colorectal, liver, breast, urinary bladder, cervical, and endometrial cancer. In several types of cancer TRPM4 is overexpressed and contributes to cancer hallmark functions such as increased proliferation and migration and cell cycle shift. Hence, TRPM4 is a potential prognostic cancer marker and a promising anticancer drug target candidate. Currently, the underlying mechanism by which TRPM4 contributes to cancer hallmark functions is under investigation. TRPM4 is a Ca²⁺-activated monovalent cation channel, and its ion conductivity can decrease intracellular Ca²⁺ signaling. Furthermore, TRPM4 can interact with different partner proteins. However, the lack of potent and specific TRPM4 inhibitors has delayed the investigations of TRPM4. In this review, we summarize the potential mechanisms of action and discuss new small molecule TRPM4 inhibitors, as well as the TRPM4 antibody, M4P. Additionally, we provide an overview of TRPM4 in human cancer and discuss TRPM4 as a diagnostic marker and anticancer drug target.

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.

Hemorrhagic Transformation After Tissue Plasminogen Activator Treatment in Acute Ischemic Stroke

Hemorrhagic transformation (HT) is a common complication after thrombolysis with recombinant tissue-type plasminogen activator (rt-PA) in ischemic stroke. In this article, recent research progress of HT in vivo and in vitro studies was reviewed. We have discussed new potential mechanisms and possible experimental models of HT development, as well as possible biomarkers and treatment methods. Meanwhile, we compared and analyzed rodent models, large animal models and in vitro BBB models of HT, and the limitations of these models were discussed. The molecular mechanism of HT was investigated in terms of BBB disruption, rt-PA neurotoxicity and the effect of neuroinflammation, matrix metalloproteinases, reactive oxygen species. The clinical features to predict HT were represented including blood biomarkers and clinical factors. Recent progress in neuroprotective strategies to improve HT after stroke treated with rt-PA is outlined. Further efforts need to be made to reduce the risk of HT after rt-PA therapy and improve the clinical prognosis of patients with ischemic stroke.

TRPM4 non-selective cation channel in human atrial fibroblast growth

Cardiac fibroblasts play an important role in cardiac matrix turnover and are involved in cardiac fibrosis development. Ca²⁺ is a driving belt in this phenomenon. This study evaluates the functional expression and contribution of the Ca²⁺-activated channel TRPM4 in atrial fibroblast phenotype. Molecular and electrophysiological investigations were conducted in human atrial fibroblasts in primary culture and in atrial fibroblasts obtained from wild-type and transgenic mice with disrupted Trpm4 gene (Trpm4^-/-). A typical TRPM4 current was recorded on human cells (equal selectivity for Na⁺ and K⁺, activation by internal Ca²⁺, voltage sensitivity, conductance of 23.2 pS, inhibition by 9-phenanthrol (IC₅₀ = 6.1 × 10^-6 mol L^-1)). Its detection rate was 13% on patches at days 2-4 in culture but raised to 100% on patches at day 28. By the same time, a cell growth was observed. This growth was smaller when cells were maintained in the presence of 9-phenanthrol. Similar cell growth was measured on wild-type mice atrial fibroblasts during culture. However, this growth was minimized on Trpm4^-/- mice fibroblasts compared to control animals. In addition, the expression of alpha smooth muscle actin increased during culture of atrial fibroblasts from wild-type mice. This was not observed in Trpm4^-/- mice fibroblasts. It is concluded that TRPM4 participates in fibroblast growth and could thus be involved in cardiac fibrosis.

Steroids and TRP Channels: A Close Relationship

Transient receptor potential (TRP) channels are remarkable transmembrane protein complexes that are essential for the physiology of the tissues in which they are expressed. They function as non-selective cation channels allowing for the signal transduction of several chemical, physical and thermal stimuli and modifying cell function. These channels play pivotal roles in the nervous and reproductive systems, kidney, pancreas, lung, bone, intestine, among others. TRP channels are finely modulated by different mechanisms: regulation of their function and/or by control of their expression or cellular/subcellular localization. These mechanisms are subject to being affected by several endogenously-produced compounds, some of which are of a lipidic nature such as steroids. Fascinatingly, steroids and TRP channels closely interplay to modulate several physiological events. Certain TRP channels are affected by the typical genomic long-term effects of steroids but others are also targets for non-genomic actions of some steroids that act as direct ligands of these receptors, as will be reviewed here.

TRP Channels as Emerging Therapeutic Targets for Neurodegenerative Diseases

The development of treatment for neurodegenerative diseases (NDs) such as Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, and amyotrophic lateral sclerosis is facing medical challenges due to the increasingly aging population. However, some pharmaceutical companies have ceased the development of therapeutics for NDs, and no new treatments for NDs have been established during the last decade. The relationship between ND pathogenesis and risk factors has not been completely elucidated. Herein, we review the potential involvement of transient receptor potential (TRP) channels in NDs, where oxidative stress and disrupted Ca²⁺ homeostasis consequently lead to neuronal apoptosis. Reactive oxygen species (ROS) -sensitive TRP channels can be key risk factors as polymodal sensors, since progressive late onset with secondary pathological damage after initial toxic insult is one of the typical characteristics of NDs. Recent evidence indicates that the dysregulation of TRP channels is a missing link between disruption of Ca²⁺ homeostasis and neuronal loss in NDs. In this review, we discuss the latest findings regarding TRP channels to provide insights into the research and quests for alternative therapeutic candidates for NDs. As the structures of TRP channels have recently been revealed by cryo-electron microscopy, it is necessary to develop new TRP channel antagonists and reevaluate existing drugs.

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

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

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.

Expression of Taste Receptor 2 Subtypes in Human Testis and Sperm

Taste receptors (TASRs) are expressed not only in the oral cavity but also throughout the body, thus suggesting that they may play different roles in organ systems beyond the tongue. Recent studies showed the expression of several TASRs in mammalian testis and sperm, indicating an involvement of these receptors in male gametogenesis and fertility. This notion is supported by an impaired reproductive phenotype of mouse carrying targeted deletion of taste receptor genes, as well as by a significant correlation between human semen parameters and specific polymorphisms of taste receptor genes. To better understand the biological and thus clinical significance of these receptors for human reproduction, we analyzed the expression of several members of the TAS2Rs family of bitter receptors in human testis and in ejaculated sperm before and after in vitro selection and capacitation. Our results provide evidence for the expression of TAS2R genes, with TAS2R14 being the most expressed bitter receptor subtype in both testis tissue and sperm cells, respectively. In addition, it was observed that in vitro capacitation significantly affects both the expression and the subcellular localization of these receptors in isolated spermatozoa. Interestingly, α-gustducin and α-transducin, two Gα subunits expressed in taste buds on the tongue, are also expressed in human spermatozoa; moreover, a subcellular redistribution of both G protein α-subunits to different sub-compartments of sperm was registered upon in vitro capacitation. Finally, we shed light on the possible downstream transduction pathway initiated upon taste receptor activation in the male reproductive system. Performing ultrasensitive droplets digital PCR assays to quantify RNA copy numbers of a distinct gene, we found a significant correlation between the expression of TAS2Rs and TRPM5 (r = 0.87), the cation channel involved in bitter but also sweet and umami taste transduction in taste buds on the tongue. Even if further studies are needed to clarify the precise functional role of taste receptors for successful reproduction, the presented findings significantly extend our knowledge of the biological role of TAS2Rs for human male fertility.

TRPM4 and TRPM5 Channels Share Crucial Amino Acid Residues for Ca2+ Sensitivity but Not Significance of PI(4,5)P2

Transient receptor potential melastatin member 4 (TRPM4) and 5 (TRPM5) channels are Ca²⁺-activated nonselective cation channels. Intracellular Ca²⁺ is the most important regulator for them to open, though PI(4,5)P₂, a membrane phosphoinositide, has been reported to regulate their Ca²⁺-sensitivities. We previously reported that negatively-charged amino acid residues near and in the TRP domain are necessary for the normal Ca²⁺ sensitivity of TRPM4. More recently, a cryo-electron microscopy structure of Ca²⁺-bound (but closed) TRPM4 was reported, proposing a Ca²⁺-binding site within an intracellular cavity formed by S2 and S3. Here, we examined the functional effects of mutations of the amino acid residues related to the proposed Ca²⁺-binding site on TRPM4 and also TRPM5 using mutagenesis and patch clamp techniques. The mutations of the amino acid residues of TRPM4 and TRPM5 reduced their Ca²⁺-sensitivities in a similar way. On the other hand, intracellular applications of PI(4,5)P₂ recovered Ca²⁺-sensitivity of desensitized TRPM4, but its effect on TRPM5 was negligible. From these results, the Ca²⁺-binding sites of TRPM4 and TRPM5 were shown to be formed by the same amino acid residues by functional analyses, but the impact of PI(4,5)P₂ on the regulation of TRPM5 seemed to be smaller than that on the regulation of TRPM4.

Is there any difference between endometrial hyperplasia and endometrial carcinoma in terms of expression of TRPM2 and TRPM7 ion channels?

Background/aim

This study compared TRPM2 and TRPM7 ion channel gene expression and immunohistochemical staining in endometrial hyperplasia and endometrium adenocarcinoma.

Materials and methods

Sections were taken from paraffin blocks of 120 patients who were divided into 6 groups as follows: G1 (n = 20), proliferative endometrium (PE); G2 (n = 20), EH without atypia; G3 (n = 20), EH with atypia; G4 (n = 20), stage 1A, grade 1 EC; G5 (n = 20), stage 1A, grade 2 EC; and G6 (n = 20), stage 1A, grade 3 EC. TRPM2 and TRPM7 genes were analyzed with qRT-PCR in paraffin-embedded tissue samples. Under light microscopy, TRPM2 and TRPM7 immunostaining scores of the samples taken from polylysine slides were evaluated.

Results

Compared to G1, TRPM2 mRNA gene expression was significantly downregulated in G3 and G5. TRPM2 immunoreactivity scores were similar in all groups. TRPM7 mRNA gene expression was significantly downregulated in G2, G3, and G6 when compared to G1. TRPM7 immunoreactivity scores were similar in G1, G2, and G3, but significantly decreased in G4, G5, and G6

Conclusion

Reduction in TRPM7 ion channel activity may be a progression marker for endometrial hyperplasia regardless of the atypical criteria.

Sodium channel TRPM4 and sodium/calcium exchangers (NCX) cooperate in the control of Ca2+-induced mucin secretion from goblet cells

Regulated mucin secretion is essential for the formation of the mucus layer that protects the underlying epithelial cells from foreign particles. Alterations in the quantity or quality of secreted mucins are therefore detrimental to airway and colon physiology. Based on various biochemical assays in several human cell lines, we report here that Na⁺/Ca²⁺ exchanger 2 (NCX2) works in conjunction with transient receptor potential cation channel subfamily M member 4 (TRPM4), and perhaps TRPM5, Na⁺ channels to control Ca²⁺-mediated secretion of both mucin 2 (MUC2) and MUC5AC from HT29-18N2 colonic cancer cells. Differentiated normal bronchial epithelial (NHBE) cells and tracheal cells from patients with cystic fibrosis (CFT1-LC3) expressed only TRPM4 and all three isoforms of NCXs. Blocking the activity of TRPM4 or NCX proteins abrogated MUC5AC secretion from NHBE and CFT1-LC3 cells. Altogether, our findings reveal that NCX and TRPM4/TRPM5 are both required for mucin secretion. We therefore propose that these two proteins could be potential pharmacological targets to control mucus-related pathologies such as cystic fibrosis.

Transient receptor potential (TRP) channels: a metabolic TR(i)P to obesity prevention and therapy

Cellular transport of ions, especially by ion channels, regulates physiological function. The transient receptor potential (TRP) channels, with 30 identified so far, are cation channels with high calcium permeability. These ion channels are present in metabolically active tissues including adipose tissue, liver, gastrointestinal tract, brain (hypothalamus), pancreas and skeletal muscle, which suggests a potential role in metabolic disorders including obesity. TRP channels have potentially important roles in adipogenesis, obesity development and its prevention and therapy because of their physiological properties including calcium permeability, thermosensation and taste perception, involvement in cell metabolic signalling and hormone release. This wide range of actions means that organ-specific actions are unlikely, thus increasing the possibility of adverse effects. Delineation of responses to TRP channels has been limited by the poor selectivity of available agonists and antagonists. Food constituents that can modulate TRP channels are of interest in controlling metabolic status. TRP vanilloid 1 channels modulated by capsaicin have been the most studied, suggesting that this may be the first target for effective pharmacological modulation in obesity. This review shows that most of the TRP channels are potential targets to reduce metabolic disorders through a range of mechanisms.

Flufenamic acid inhibits secondary hemorrhage and BSCB disruption after spinal cord injury

Acute spinal cord injury (SCI) induces secondary hemorrhage and initial blood-spinal cord barrier (BSCB) disruption. The transient receptor potential melastatin 4 (Trpm4) together with sulfonylurea receptor 1 (Sur1) forms the Sur1-Trpm4 channel complex. The up-regulation of Sur1-Trpm4 after injury plays a crucial role in secondary hemorrhage, which is the most destructive mechanism in secondary injuries of the central nervous system (CNS). The matrix metalloprotease (MMP)-mediated disruption of the BSCB leads to an inflammatory response, neurotoxin production and neuronal cell apoptosis. Thus, preventing secondary hemorrhage and BSCB disruption should be an important goal of therapeutic interventions in SCI.

Methods: Using a moderate contusion injury model at T10 of the spinal cord, flufenamic acid (FFA) was injected intraperitoneally 1 h after SCI and then continuously once per day for one week.

Results: Trpm4 expression is highly up-regulated in capillaries 1 d after SCI. Treatment with flufenamic acid (FFA) inhibited Trpm4 expression, secondary hemorrhage, and capillary fragmentation and promoted angiogenesis. In addition, FFA significantly inhibited the expression of MMP-2 and MMP-9 at 1 d after SCI and significantly attenuated BSCB disruption at 1 d and 3 d after injury. Furthermore, we found that FFA decreased the hemorrhage- and BSCB disruption-induced activation of microglia/macrophages and was associated with smaller lesions, decreased cavity formation, better myelin preservation and less reactive gliosis. Finally, FFA protected motor neurons and improved locomotor functions after SCI.

Conclusion: This study indicates that FFA improves functional recovery, in part, due to the following reasons: (1) it inhibits the expression of Trpm4 to reduce the secondary hemorrhage; and (2) it inhibits the expression of MMP-2 and MMP-9 to block BSCB disruption. Thus, the results of our study suggest that FFA may represent a potential therapeutic agent for promoting functional recovery.

Ethanol's Effects on Transient Receptor Potential Channel Expression in Brain Microvascular Endothelial Cells

Ethanol (EtOH), the main ingredient in alcoholic beverages, is well known for its behavioral, physiological, and immunosuppressive effects. There is evidence that EtOH acts through protein targets to exert its physiological effects; however, the mechanisms underlying EtOH's effects on inflammatory processes, particularly at the blood-brain barrier (BBB), are still poorly understood. Transient receptor potential (TRP) channels, the vanguards of human sensory systems, are novel molecular receptors significantly affected by EtOH, and are heavily expressed in brain microvascular endothelial cells (BMVECs), one of the cellular constituents of the BBB. EtOH's actions on endothelial TRP channels could affect intracellular Ca²⁺ and Mg²⁺ dynamics, which mediate leukocyte adhesion to endothelial cells and endothelial permeability at the BBB, thus altering immune and inflammatory responses. We examined the basal expression profiles of all 29 known mammalian TRP channels in mouse BMVECs and determined both EtOH concentration- and time-dependent effects on TRP expression using a PCR array. We also generated an in vitro BBB model to examine the involvement of a chosen TRP channel, TRP melastatin 7 (TRPM7), in EtOH-mediated alteration of BBB permeability. With the exception of the akyrin subfamily, members of five TRP subfamilies were expressed in mouse BMVECs, and their expression levels were modulated by EtOH in a concentration-dependent manner. In the in vitro BBB model, TRPM7 antagonists further enhanced EtOH-mediated alteration of BBB permeability. Because of the diversity of TRP channels in BMVECs that regulate cellular processes, EtOH can affect Ca²⁺/Mg²⁺ signaling, immune responses, lysosomal functions as well as BBB integrity.

TRPM5 in the battle against diabetes and obesity

TRPM5 is a non-selective monovalent cation channel activated by increases in intracellular Ca²⁺ . It has a distinct expression pattern: expression is detected in chemosensitive tissues from solitary chemosensory cells to the taste receptor cells and in pancreatic β-cells. The role of TRPM5 has been investigated with the use of knockout mouse models. Trpm5^-/- mice have a lack of type II taste perception and show reduced glucose-induced insulin secretion. Expression levels of TRPM5 are reduced in obese, leptin-signalling-deficient mice, and mutations in TRPM5 have been associated with type II diabetes and metabolic syndrome. In this review, we aim to give an overview of the activation, selectivity, modulation and physiological roles of TRPM5.

TRPM4 and TRPM5 are both required for normal signaling in taste receptor cells

Peripheral taste receptor cells use multiple signaling pathways to transduce taste stimuli into output signals that are sent to the brain. Transient receptor potential melastatin 5 (TRPM5), a sodium-selective TRP channel, functions as a common downstream component in sweet, bitter, and umami signaling pathways. In the absence of TRPM5, mice have a reduced, but not abolished, ability to detect stimuli, suggesting that a TRPM5-independent pathway also contributes to these signals. Here, we identify a critical role for the sodium-selective TRP channel TRPM4 in taste transduction. Using live cell imaging and behavioral studies in KO mice, we show that TRPM4 and TRPM5 are both involved in taste-evoked signaling. Loss of either channel significantly impairs taste, and loss of both channels completely abolishes the ability to detect bitter, sweet, or umami stimuli. Thus, both TRPM4 and TRPM5 are required for transduction of taste stimuli.

Actions and Regulation of Ionotropic Cannabinoid Receptors

Almost three decades have passed since the identification of the two specific metabotropic receptors mediating cannabinoid pharmacology. Thereafter, many cannabinoid effects, both at central and peripheral levels, have been well documented and characterized. However, numerous evidences demonstrated that these pharmacological actions could not be attributable solely to the activation of CB1 and CB2 receptors since several important cannabimimetic actions have been found in biological systems lacking CB1 or CB2 gene such as in specific cell lines or transgenic mice. It is now well accepted that, beyond their receptor-mediated effects, these molecules can act also via CB1/CB2-receptor-independent mechanism. Cannabinoids have been demonstrated to modulate several voltage-gated channels (including Ca²⁺, Na⁺, and various type of K⁺ channels), ligand-gated ion channels (i.e., GABA, glycine), and ion-transporting membranes proteins such as transient potential receptor class (TRP) channels. The first direct, cannabinoid receptor-independent interaction was reported on the function of serotonin 5-HT₃ receptor-ion channel complex. Similar effects were reported also on the other above mentioned ion channels. In the early ninety, studies searching for endogenous modulators of L-type Ca²⁺ channels identified anandamide as ligand for L-type Ca²⁺ channel. Later investigations indicated that other types of Ca²⁺ currents are also affected by endocannabinoids, and, in the late ninety, it was discovered that endocannabinoids activate the vanilloid receptor subtype 1 (TRPV1), and nowadays, it is known that (endo)cannabinoids gate at least five distinct TRP channels. This chapter focuses on cannabinoid regulation of ion channels and lays special emphasis on their action at transient receptor channels.

TRPM channels as potential therapeutic targets against pro-inflammatory diseases

The immune system protects our body against foreign pathogens. However, if it overshoots or turns against itself, pro-inflammatory diseases, such as rheumatoid arthritis, inflammatory bowel disease, or diabetes develop. Ions, the most basic signaling molecules, shape intracellular signaling cascades resulting in immune cell activation and subsequent immune responses. Mutations in ion channels required for calcium signaling result in human immunodeficiencies and highlight those ion channels as valued targets for therapies against pro-inflammatory diseases. Signaling pathways regulated by melastatin-like transient receptor potential (TRPM) cation channels also play crucial roles in calcium signaling and leukocyte physiology, affecting phagocytosis, degranulation, chemokine and cytokine expression, chemotaxis and invasion, as well as lymphocyte development and proliferation. Therefore, this review discusses their regulation, possible interactions and whether they can be exploited as targets for therapeutic approaches to pro-inflammatory diseases.

Ion channels in control of pancreatic stellate cell migration

Pancreatic stellate cells (PSCs) play a critical role in the progression of pancreatic ductal adenocarcinoma (PDAC). Once activated, PSCs support proliferation and metastasis of carcinoma cells. PSCs even co-metastasise with carcinoma cells. This requires the ability of PSCs to migrate. In recent years, it has been established that almost all "hallmarks of cancer" such as proliferation or migration/invasion also rely on the expression and function of ion channels. So far, there is only very limited information about the function of ion channels in PSCs. Yet, there is growing evidence that ion channels in stromal cells also contribute to tumor progression. Here we investigated the function of KCa3.1 channels in PSCs. KCa3.1 channels are also found in many tumor cells of different origin. We revealed the functional expression of KCa3.1 channels by means of Western blot, immunofluorescence and patch clamp analysis. The impact of KCa3.1 channel activity on PSC function was determined with live-cell imaging and by measuring the intracellular Ca2+ concentration ([Ca2+]i). KCa3.1 channel blockade or knockout prevents the stimulation of PSC migration and chemotaxis by reducing the [Ca2+]i and calpain activity. KCa3.1 channels functionally cooperate with TRPC3 channels that are upregulated in PDAC stroma. Knockdown of TRPC3 channels largely abolishes the impact of KCa3.1 channels on PSC migration. In summary, our results clearly show that ion channels are crucial players in PSC physiology and pathophysiology.

Potential role of melastatin-related transient receptor potential cation channel subfamily M gene expression in the pathogenesis of urinary bladder cancer

Urinary bladder cancer is one of the most common malignancies of the urinary tract. Ion channels and calcium homeostasis are involved in almost all basic cellular mechanisms. The transient receptor potential cation channel subfamily M (TRPM) takes its name from the melastatin protein, which is classified as potential tumor suppressor. To the best of our knowledge, there have been no previous studies in the literature investigating the role of these ion channels in bladder cancer. The present study aimed to determine whether bladder cancer is associated with mRNA expression levels of TRPM ion channel genes, and whether there is the potential to conduct further studies to establish novel treatment modalities. The present study included a total of 47 subjects, of whom 40 were bladder cancer patients and 7 were controls. Following the histopathological evaluation for bladder carcinoma, the mRNA and protein expression of TRPM were examined by reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and immunohistochemistry in tumor and normal tissues, in order to determine whether there is a difference in the expression of these channels in tumor and normal tissues. Immunoreactivity for TRPM2, TRPM4, TRPM7 and TRPM8 was observed in epithelial bladder cells in the two groups. RT-qPCR revealed a significant increase in TRPM7 expression in bladder cancer tissue compared to the controls (healthy bladder tissue), whereas no differences in TRPM2 or TRPM4 expression levels were observed. There were significant reductions in the expression levels of TRPM5 and TRPM8 in bladder cancer tissues. In the present study, the effects of TRP ion channels on the formation of bladder cancer was investigated. This study is instructive for TRPM2, TRPM4, TRPM5, TRPM7 and TRPM8 and their therapeutic role in bladder cancer. The results support the fact that these gens can be novel targets and can also be tested for during the treatment of bladder cancer.

Regulation of thermoTRPs by lipids

The family of Transient Receptor Potential (TRP) ion channels is constituted by 7 subfamilies among which are those that respond to temperature, the thermoTRPs. These channels are versatile molecules of a polymodal nature that have been shown to be modulated in various fashions by molecules of a lipidic nature. Some of these molecules interact directly with the channels on specific regions of their structures and some of these promote changes in membrane fluidity or modify their gating properties in response to their agonists. Here, we have discussed how some of these lipids regulate the activity of thermoTRPs and included some of the available evidence for the molecular mechanisms underlying their effects on these channels.

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.

Honing in on the ATP Release Channel in Taste Cells

Studies over the last 8 years have identified 3 potential channels that appear to release ATP from Type II cells in response to taste stimuli. These studies have taken different methodological approaches but have all provided data supporting their candidate channel as the ATP release channel. These potential channels include Pannexin 1, Connexins (30 and/or 43), and most recently, the Calhm1 channel. Two papers in this issue of Chemical Senses provide compelling new evidence that Pannexin 1 is not the ATP release channel. Tordoff et al. did a thorough behavioral analysis of the Pannexin1 knock out mouse and found that these animals have the same behavioral responses as wild type mice for 7 different taste stimuli that were tested. Vandenbeuch et al. presented an equally thorough analysis of the gustatory nerve responses in the Pannexin1 knock out mouse and found no differences compared with controls. Thus when the role of Pannexin 1 is analyzed at the systems level, it is not required for normal taste perception. Further studies are needed to determine the role of this hemichannel in taste cells.

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+).

Novel coupling between TRPC-like and KNa channels modulates low threshold spike-induced afterpotentials in rat thalamic midline neurons

Neurons in thalamic midline and paraventricular nuclei (PVT) display a unique slow afterhyperpolarizing potential (sAHP) following the low threshold spike (LTS) generated by activation of their low voltage Ca(2+) channels. We evaluated the conductances underlying this sAHP using whole-cell patch-clamp recordings in rat brain slice preparations. Initial observations recorded in the presence of TTX revealed a marked dependency of the LTS-induced sAHP on extracellular Na(+): replacing Na(+) with TRIS(+) in the external medium eliminated the LTS-induced sAHP; substitution of Na(+) with either Li(+) or choline(+) in the external medium resulted in a gradual loss of the sAHP and its replacement with a prolonged slow afterdepolarizing potential (sADP). The LTS-induced sAHP was reduced by quinidine and potentiated by loxapine, suggesting involvement of KNa-like channels. Canonical transient receptor potential (TRPC) channels were considered the source for Na(+) based on observations that the sAHP was suppressed by nonselective TRPC channel blockers (2-APB, flufenamic acid and ML204) but unchanged in the presence of TRPV1 channel blocker (SB-366791). In addition, after replacement of Na(+) with Li(+), the isolated LTS-induced sADP was significantly suppressed in the presence of 2-APB or ML204, after replacement of extracellular Ca(2+) with Sr(2+), and by intracellular Ca(2+) chelation with EGTA, data that collectively suggest involvement of Ca(2+)-activated TRPC-like conductances containing TRPC4/5 subunits. The isolated LTS-induced sADP also exhibited a strong voltage dependency, decreasing at hyperpolarizing potentials, further support for involvement of TRPC4/5 subunits. This sADP exhibited neurotransmitter receptor sensitivity, with suppression by 5-CT, a 5-HT7 receptor agonist, and enhancement by the neuropeptide orexin A. These data suggest that LTS-induced slow afterpotentials reflect a simultaneous interplay between KNa and TRPC-like conductances, novel for midline thalamic neurons.

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.

Riluzole suppresses postinhibitory rebound in an excitatory motor neuron of the medicinal leech

Postinhibitory rebound (PIR) is an intrinsic property often exhibited by neurons involved in generating rhythmic motor behaviors. Cell DE-3, a dorsal excitatory motor neuron in the medicinal leech exhibits PIR responses that persist for several seconds following the offset of hyperpolarizing stimuli and are suppressed in reduced Na(+) solutions or by Ca(2+) channel blockers. The long duration and Na(+) dependence of PIR suggest a possible role for persistent Na(+) current (I NaP). In vertebrate neurons, the neuroprotective agent riluzole can produce a selective block of I NaP. This study demonstrates that riluzole inhibits cell DE-3 PIR in a concentration- and Ca(2+)-dependent manner. In 1.8 mM Ca(2+) solution, 50-100 µM riluzole selectively blocked the late phase of PIR, an effect similar to that of the neuromodulator serotonin. However, 200 µM riluzole blocked both the early and late phases of PIR. Increasing extracellular Ca(2+) to 10 mM strengthened PIR, but high riluzole concentrations continued to suppress both phases of PIR. These results indicate that riluzole may suppress PIR via a nonspecific inhibition of Ca(2+) conductances and suggest that a Ca(2+)-activated nonspecific current (I(CAN)), rather than I NaP, may underlie the Na(+)-dependent component of PIR.

P2Y1 receptor-mediated potentiation of inspiratory motor output in neonatal rat in vitro

PreBötzinger complex inspiratory rhythm-generating networks are excited by metabotropic purinergic receptor subtype 1 (P2Y1R) activation. Despite this, and the fact that inspiratory MNs express P2Y1Rs, the role of P2Y1Rs in modulating motor output is not known for any MN pool. We used rhythmically active brainstem-spinal cord and medullary slice preparations from neonatal rats to investigate the effects of P2Y1R signalling on inspiratory output of phrenic and XII MNs that innervate diaphragm and airway muscles, respectively. MRS2365 (P2Y1R agonist, 0.1 mm) potentiated XII inspiratory burst amplitude by 60 ± 9%; 10-fold higher concentrations potentiated C4 burst amplitude by 25 ± 7%. In whole-cell voltage-clamped XII MNs, MRS2365 evoked small inward currents and potentiated spontaneous EPSCs and inspiratory synaptic currents, but these effects were absent in TTX at resting membrane potential. Voltage ramps revealed a persistent inward current (PIC) that was attenuated by: flufenamic acid (FFA), a blocker of the Ca(2+)-dependent non-selective cation current ICAN; high intracellular concentrations of BAPTA, which buffers Ca(2+) increases necessary for activation of ICAN; and 9-phenanthrol, a selective blocker of TRPM4 channels (candidate for ICAN). Real-time PCR analysis of mRNA extracted from XII punches and laser-microdissected XII MNs revealed the transcript for TRPM4. MRS2365 potentiated the PIC and this potentiation was blocked by FFA, which also blocked the MRS2365 potentiation of glutamate currents. These data suggest that XII MNs are more sensitive to P2Y1R modulation than phrenic MNs and that the P2Y1R potentiation of inspiratory output occurs in part via potentiation of TRPM4-mediated ICAN, which amplifies inspiratory inputs.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Structure and function of TMEM16 proteins (anoctamins)

TMEM16 proteins, also known as anoctamins, are involved in a variety of functions that include ion transport, phospholipid scrambling, and regulation of other membrane proteins. The first two members of the family, TMEM16A (anoctamin-1, ANO1) and TMEM16B (anoctamin-2, ANO2), function as Ca2+-activated Cl- channels (CaCCs), a type of ion channel that plays important functions such as transepithelial ion transport, smooth muscle contraction, olfaction, phototransduction, nociception, and control of neuronal excitability. Genetic ablation of TMEM16A in mice causes impairment of epithelial Cl- secretion, tracheal abnormalities, and block of gastrointestinal peristalsis. TMEM16A is directly regulated by cytosolic Ca2+ as well as indirectly by its interaction with calmodulin. Other members of the anoctamin family, such as TMEM16C, TMEM16D, TMEM16F, TMEM16G, and TMEM16J, may work as phospholipid scramblases and/or ion channels. In particular, TMEM16F (ANO6) is a major contributor to the process of phosphatidylserine translocation from the inner to the outer leaflet of the plasma membrane. Intriguingly, TMEM16F is also associated with the appearance of anion/cation channels activated by very high Ca2+ concentrations. Furthermore, a TMEM16 protein expressed in Aspergillus fumigatus displays both ion channel and lipid scramblase activity. This finding suggests that dual function is an ancestral characteristic of TMEM16 proteins and that some members, such as TMEM16A and TMEM16B, have evolved to a pure channel function. Mutations in anoctamin genes (ANO3, ANO5, ANO6, and ANO10) cause various genetic diseases. These diseases suggest the involvement of anoctamins in a variety of cell functions whose link with ion transport and/or lipid scrambling needs to be clarified.