ß-Lactam TRPM8 Antagonists Derived from Phe-Phenylalaninol Conjugates: Structure-Activity Relationships and Antiallodynic Activity.

The protein transient receptor potential melastatin type 8 (TRPM8), a non-selective, calcium (Ca2+)-permeable ion channel is implicated in several pathological conditions, including neuropathic pain states. In our previous research endeavors, we have identified ß-lactam derivatives with high hydrophobic character that exhibit potent and selective TRPM8 antagonist activity. This work describes the synthesis of novel derivatives featuring C-terminal amides and diversely substituted N'-terminal monobenzyl groups in an attempt to increase the total polar surface area (TPSA) in this family of compounds. The primary goal was to assess the influence of these substituents on the inhibition of menthol-induced cellular Ca2+ entry, thereby establishing critical structure-activity relationships. While the substitution of the tert-butyl ester by isobutyl amide moieties improved the antagonist activity, none of the N'-monobencyl derivatives, regardless of the substituent on the phenyl ring, achieved the activity of the model dibenzyl compound. The antagonist potency of the most effective compounds was subsequently verified using Patch-Clamp electrophysiology experiments. Furthermore, we evaluated the selectivity of one of these compounds against other members of the transient receptor potential (TRP) ion channel family and some receptors connected to peripheral pain pathways. This compound demonstrated specificity for TRPM8 channels. To better comprehend the potential mode of interaction, we conducted docking experiments to uncover plausible binding sites on the functionally active tetrameric protein. While the four main populated poses are located by the pore zone, a similar location to that described for the N-(3-aminopropyl)-2-[(3-methylphenyl)methoxy]-N-(2-thienylmethyl)-benzamide (AMTB) antagonist cannot be discarded. Finally, in vivo experiments, involving a couple of selected compounds, revealed significant antinociceptive activity within a mice model of cold allodynia induced by oxaliplatin (OXA).

Clotrimazole-Based Modulators of the TRPM3 Ion Channel Reveal Narrow Structure-Activity Relationship.

TRPM3 is an ion channel that is highly expressed in nociceptive neurons and plays a key role in pain perception. In the presence of the endogenous TRPM3 ligand, pregnenolone sulfate (PS), the antifungal compound clotrimazole (Clt) augments Ca2+ signaling and opens a non-canonical pore, permeable to Na+, which aggravates TRPM3-induced pain. To date, little is known about structural features that govern the Clt modulatory effect of TRPM3. Here, we synthesized and evaluated several Clt analogues in order to gain insights into their structure-activity relationship. Our results reveal a tight SAR with the three phenyl rings on the trityl moiety being essential for the activity, as well as the presence of fluorine or chlorine substituents on the trityl group. Imidazole as a heterocycle is also necessary for activity. Interestingly, we identified a pentafluoro-trityl analogue (29a) that is able to act as a TRPM3 agonist in the absence of PS. The compounds we report in this work will be useful tools for the further study of TRPM3 modulation and its effect on pain perception.

TRPM4 inhibition by meclofenamate suppresses Ca2+-dependent triggered arrhythmias.

AIMS: Cardiac arrhythmias are a major factor in the occurrence of morbidity and sudden death in patients with cardiovascular disease. Disturbances of Ca2+ homeostasis in the heart contribute to the initiation and maintenance of cardiac arrhythmias. Extrasystolic increases in intracellular Ca2+ lead to delayed afterdepolarizations and triggered activity, which can result in heart rhythm abnormalities. It is being suggested that the Ca2+-activated nonselective cation channel TRPM4 is involved in the aetiology of triggered activity, but the exact contribution and in vivo significance are still unclear. METHODS AND RESULTS: In vitro electrophysiological and calcium imaging technique as well as in vivo intracardiac and telemetric electrocardiogram measurements in physiological and pathophysiological conditions were performed. In two distinct Ca2+-dependent proarrhythmic models, freely moving Trpm4-/- mice displayed a reduced burden of cardiac arrhythmias. Looking further into the specific contribution of TRPM4 to the cellular mechanism of arrhythmias, TRPM4 was found to contribute to a long-lasting Ca2+ overload-induced background current, thereby regulating cell excitability in Ca2+ overload conditions. To expand these results, a compound screening revealed meclofenamate as a potent antagonist of TRPM4. In line with the findings from Trpm4-/- mice, 10 µM meclofenamate inhibited the Ca2+ overload-induced background current in ventricular cardiomyocytes and 15 mg/kg meclofenamate suppressed catecholaminergic polymorphic ventricular tachycardia-associated arrhythmias in a TRPM4-dependent manner. CONCLUSION: The presented data establish that TRPM4 represents a novel target in the prevention and treatment of Ca2+-dependent triggered arrhythmias.

Loratadine, an antihistaminic drug, suppresses the proliferation of endometrial stromal cells by inhibition of TRPV2.

The transient receptor potential (TRP) channel TRPV2 is widely expressed in a variety of different cell types and tissues. However, elucidating the exact biological functions of TRPV2 is significantly hampered by the lack of selective pharmacological tools to modulate channel activity in vitro and in vivo. This study aimed to identify new compounds that modify TRPV2 activity via the use of a plate-based calcium imaging approach to screen a drug repurposing library. Three antihistaminic drugs, loratadine, astemizole and clemizole were identified to reduce calcium-influx evoked by the TRPV2 agonist tetrahydrocannabivarin in HEK293 cells expressing murine TRPV2. Using single-cell calcium-microfluorimetry and whole-cell patch clamp recordings, we further confirmed that all three compounds induced a concentration-dependent block of TRPV2-mediated Ca2+ influx and whole-cell currents, with loratadine being the most potent antagonist of TRPV2. Moreover, this study demonstrated that loratadine was able to block both the human and mouse TRPV2 orthologs, without inhibiting the activity of other closely related members of the TRPV superfamily. Finally, loratadine inhibited TRPV2-dependent responses in a primary culture of mouse endometrial stromal cells and attenuated cell proliferation and migration in in vitro cell proliferation and wound healing assays. Taken together, our study revealed that the antihistaminic drugs loratadine, astemizole and clemizole target TRPV2 in a concentration-dependent manner. The identification of these antihistaminic drugs as blockers of TRPV2 may form a new starting point for the synthesis of more potent and selective TRPV2 antagonists, which could further lead to the unravelling of the physiological role of the channel.

Partial Agonistic Actions of Sex Hormone Steroids on TRPM3 Function.

Sex hormone steroidal drugs were reported to have modulating actions on the ion channel TRPM3. Pregnenolone sulphate (PS) presents the most potent known endogenous chemical agonist of TRPM3 and affects several gating modes of the channel. These includes a synergistic action of PS and high temperatures on channel opening and the PS-induced opening of a noncanonical pore in the presence of other TRPM3 modulators. Moreover, human TRPM3 variants associated with neurodevelopmental disease exhibit an increased sensitivity for PS. However, other steroidal sex hormones were reported to influence TRPM3 functions with activating or inhibiting capacity. Here, we aimed to answer how DHEAS, estradiol, progesterone and testosterone act on the various modes of TRPM3 function in the wild-type channel and two-channel variants associated with human disease. By means of calcium imaging and whole-cell patch clamp experiments, we revealed that all four drugs are weak TRPM3 agonists that share a common steroidal interaction site. Furthermore, they exhibit increased activity on TRPM3 at physiological temperatures and in channels that carry disease-associated mutations. Finally, all steroids are able to open the noncanonical pore in wild-type and DHEAS also in mutant TRPM3. Collectively, our data provide new valuable insights in TRPM3 gating, structure-function relationships and ligand sensitivity.

AAV9-Mediated Overexpression of TRPM4 Increases the Incidence of Stress-Induced Ventricular Arrhythmias in Mice.

Ca2+ activated non-selective (CAN) cation channels have been described in cardiomyocytes since the advent of the patch clamp technique. It has been hypothesized that this type of ion channel contributes to the triggering of cardiac arrhythmias. TRPM4 is to date the only molecular candidate for a CAN cation channel in cardiomyocytes. Its significance for arrhythmogenesis in living animals remains, however, unclear. In this study, we have tested whether increased expression of wild-type (WT) TRPM4 augments the risk of arrhythmias in living mice. Overexpression of WT TRPM4 was achieved via tail vein injection of adeno-associated viral vector serotype 9 (AAV9) particles, which have been described to be relatively cardiac specific in mice. Subsequently, we performed ECG-measurements in freely moving mice to determine their in vivo cardiac phenotype. Though cardiac muscle was transduced with TRPM4 viral particles, the majority of viral particles accumulated in the liver. We did not observe any difference in arrhythmic incidents during baseline conditions. Instead, WT mice that overexpress TRPM4 were more vulnerable to develop premature ventricular ectopic beats during exercise-induced ß-adrenergic stress. Conduction abnormalities were rare and not more frequent in transduced mice compare to WT mice. Taken together, we provide evidence that overexpression of TRPM4 increases the susceptibility of living mice to stress-induced arrhythmias.

Store-independent coupling between the Secretory Pathway Ca2+ transport ATPase SPCA1 and Orai1 in Golgi stress and Hailey-Hailey disease.

The Secretory Pathway Ca2+ ATPases SPCA1 and SPCA2 transport Ca2+ and Mn2+ into the Golgi and Secretory Pathway. SPCA2 mediates store-independent Ca2+ entry (SICE) via STIM1-independent activation of Orai1, inducing constitutive Ca2+ influx in mammary epithelial cells during lactation. Here, we show that like SPCA2, also the overexpression of the ubiquitous SPCA1 induces cytosolic Ca2+ influx, which is abolished by Orai1 knockdown and occurs independently of STIM1. This process elevates the Ca2+ concentration in the cytosol and in the non-endoplasmic reticulum (ER) stores, pointing to a functional coupling between Orai1 and SPCA1. In agreement with this, we demonstrate via Total Internal Reflection Fluorescence microscopy that Orai1 and SPCA1a co-localize near the plasma membrane. Interestingly, SPCA1 overexpression also induces Golgi swelling, which coincides with translocation of the transcription factor TFE3 to the nucleus, a marker of Golgi stress. The induction of Golgi stress depends on a combination of SPCA1 activity and SICE, suggesting a role for the increased Ca2+ level in the non-ER stores. Finally, we tested whether impaired SPCA1a/Orai1 coupling may be implicated in the skin disorder Hailey-Hailey disease (HHD), which is caused by SPCA1 loss-of-function. We identified HHD-associated SPCA1a mutations that impair either the Ca2+ transport function, Orai1 activation, or both, while all mutations affect the Ca2+ content of the non-ER stores. Thus, the functional coupling between SPCA1 and Orai1 increases cytosolic and intraluminal Ca2+ levels, representing a novel mechanism of SICE that may be affected in HHD.

A Thallium-Based Screening Procedure to Identify Molecules That Modulate the Activity of Ca2+-Activated Monovalent Cation-Selective Channels.

TRPM5 functions as a calcium-activated monovalent cation-selective ion channel and is expressed in a variety of cell types. Dysfunction of this type of channel has been recently implied in cardiac arrhythmias, diabetes, and other pathologies. Therefore, a growing interest has emerged to develop the pharmacology of these ion channels. We optimized a screening assay based on the thallium flux through the TRPM5 channel and a fluorescent thallium dye as a probe for channel activity. We show that this assay is capable of identifying molecules that inhibit or potentiate calcium-activated monovalent cation-selective ion channels.

Steviol glycosides enhance pancreatic beta-cell function and taste sensation by potentiation of TRPM5 channel activity.

Steviol glycosides (SGs), such as stevioside and rebaudioside A, are natural, non-caloric sweet-tasting organic molecules, present in extracts of the scrub plant Stevia rebaudiana, which are widely used as sweeteners in consumer foods and beverages. TRPM5 is a Ca2+-activated cation channel expressed in type II taste receptor cells and pancreatic ß-cells. Here we show that stevioside, rebaudioside A and their aglycon steviol potentiate the activity of TRPM5. We find that SGs potentiate perception of bitter, sweet and umami taste, and enhance glucose-induced insulin secretion in a Trpm5-dependent manner. Daily consumption of stevioside prevents development of high-fat-diet-induced diabetic hyperglycaemia in wild-type mice, but not in Trpm5-/- mice. These results elucidate a molecular mechanism of action of SGs and identify TRPM5 as a potential target to prevent and treat type 2 diabetes.

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

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

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

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

Enhanced ß-adrenergic cardiac reserve in Trpm4⁻/⁻ mice with ischaemic heart failure.

AIMS: Heart failure (HF) is a complex syndrome characterized by critically reduced cardiac contractility and function. We have shown previously that Transient Receptor Potential Melastatin 4 protein (TRPM4) functions as a Ca(2+)-activated non-selective cation channel and constitutes a novel regulator of ventricular contractility. In healthy Trpm4-deficient (Trpm4(-/-)) mice, we observed increased cardiac contractile function after ß-adrenergic stimulation. In the current study, cardiac performance was examined in wild-type (WT) and Trpm4(-/-) mice with severe ischaemic HF. METHODS AND RESULTS: Myocardial infarction (MI) was induced in WT and Trpm4(-/-) C57Bl6/N mice by ligation of the left anterior descending artery. During the first week after MI, mortality was higher in WT mice. Both groups showed similar infarct-typical ECG patterns during follow-up period. After 10 weeks, reduced ejection fraction and severe dilatation, determined by cardiac MRI, confirmed the development of HF in both genotypes. In vivo pressure-conductance analysis revealed no differences in cardiac contractility in basal conditions. However, during ß-adrenergic stimulation, cardiac performance was significantly different between WT and Trpm4(-/-) mice. In contrast to increasing contractility in Trpm4(-/-) mice, WT mice showed a deteriorated cardiac performance. Also 30% of WT animals died during isoprenaline infusion vs. no Trpm4(-/-) mice. Infarct size, determined post mortem, was equal in WT and Trpm4(-/-) hearts. CONCLUSION: Deletion of the Trpm4 gene in mice improved survival and significantly enhanced ß-adrenergic cardiac reserve after inducing ischaemic HF. This suggests that pharmacological or genetic down-regulation of TRPM4 function might be a novel strategy in the management of HF.

Opening of an alternative ion permeation pathway in a nociceptor TRP channel.

Sensory neurons detect chemical stimuli through projections in the skin and mucosa, where several transient receptor potential (TRP) channels act as primary chemosensors. TRP channels are tetramers, and it is generally accepted that binding of ligands causes the opening of a single central cation-conducting pore. Contrary to this view, we here provide evidence for a second permeation pathway in the TRP channel TRPM3, which can be gated by combined application of endogenous neurosteroids and exogenous chemicals such as clotrimazole or several structurally related drugs. This alternative pathway is preserved following desensitization, blockade, mutagenesis and chemical modification of the central pore and enables massive Na(+) influx at negative voltages. Opening of this alternative pathway can enhance excitation of sensory neurons and thereby exacerbate TRPM3-dependent pain. Our findings indicate that a single sensory TRP channel can encompass two distinct ionotropic chemoreceptors, which may have important ramifications for TRP channel function and pharmacology.

Functional characterization of a chronic cyclophosphamide-induced overactive bladder model in mice.

AIMS: To describe a new mouse model of overactive bladder (OAB) at the histological level, pain, voiding behavior, and urodynamics, while assessing the physiological state of mice. METHODS: This paper compares the pathophysiological features of mice that received intraperitoneal injections of cyclophosphamide (CYP) (40 and 80 mg/kg - body weight) every 2 days for 7 days. Specifically, the heart rate, the body temperature, and the general activity were assessed by telemetry. The abdominal sensitivity was determined with Von Frey filaments. Voiding behavior and detrusor activity were respectively quantified by urine spotting experiments and cystometry. Hematoxylin & Eosin staining was performed to detect inflammation in tissue and NGF concentration in urine was quantified. RESULTS: Affected mice exhibit clearly an OAB characterized by an increase in the number of voiding events and an urodynamically-demonstrated detrusor overactivity associated with referred hyperalgesia. The injected mice displayed inflamed bladder, urothelial hyperplasia, and increased NGF concentration in urine in dose dependant manner. However, the physiological features of mice with CYP-induced cystitis are not changed. CONCLUSIONS: We can show that this model of chronic OAB with pain in mice fits more closely to the clinical signs of patients with OAB than the available animal models (acute and chronic) and will therefore be useful to highlight potential drug targets in genetically modified mice in the future.

TRPM3 is a nociceptor channel involved in the detection of noxious heat.

Transient receptor potential melastatin-3 (TRPM3) is a broadly expressed Ca(2+)-permeable nonselective cation channel. Previous work has demonstrated robust activation of TRPM3 by the neuroactive steroid pregnenolone sulfate (PS), but its in vivo gating mechanisms and functions remained poorly understood. Here, we provide evidence that TRPM3 functions as a chemo- and thermosensor in the somatosensory system. TRPM3 is molecularly and functionally expressed in a large subset of small-diameter sensory neurons from dorsal root and trigeminal ganglia, and mediates the aversive and nocifensive behavioral responses to PS. Moreover, we demonstrate that TRPM3 is steeply activated by heating and underlies heat sensitivity in a subset of sensory neurons. TRPM3-deficient mice exhibited clear deficits in their avoidance responses to noxious heat and in the development of inflammatory heat hyperalgesia. These experiments reveal an unanticipated role for TRPM3 as a thermosensitive nociceptor channel implicated in the detection of noxious heat.