Decreased anxiety-like behavior and Gαq/11-dependent responses in the amygdala of mice lacking TRPC4 channels

TRPC4/5 inhibitors: Phase I results and proof of concept studies

Transient receptor potential canonical (TRPC) ion channels are expressed in areas of the brain responsible for processing emotion and mood and have been implicated in the pathophysiology of internalizing disorders such as major depressive disorder and anxiety disorders. This review outlines the rationale for targeting TRPC ion channels for drug development, with specific focus on TRPC4 and TRPC5. We provide preclinical evidence that the lack of TRPC4 and TRPC5 channels or its pharmacological inhibition attenuate fear and anxiety without impairing other behaviors in mice. We also report on clinical studies of BI 1358894, a small molecule inhibitor of TRPC4/5 ion channels, demonstrating reduced psychological and physiological responses to induced anxiety/panic-like symptoms in healthy volunteers. Furthermore, we highlight an imaging study that investigated the acute effects of BI 1358894 and showed reduced activation in several brain regions involved in emotional processing. We conclude that these findings demonstrate a critical role for TRPC4 and TRPC5 in emotional processing, even though it remains an open question if the biological signatures of TRPC4/5 inhibition reported here translate into clinical efficacy and indicate that a TRPC4/5 inhibitor might provide a more effective treatment of internalizing disorders.

TRP Channels in Stroke

Ischemic stroke is a devastating disease that affects millions of patients worldwide. Unfortunately, there are no effective medications for mitigating brain injury after ischemic stroke. TRP channels are evolutionally ancient biosensors that detect external stimuli as well as tissue or cellular injury. To date, many members of the TRP superfamily have been reported to contribute to ischemic brain injury, including the TRPC subfamily (1, 3, 4, 5, 6, 7), TRPV subfamily (1, 2, 3, 4) and TRPM subfamily (2, 4, 7). These TRP channels share structural similarities but have distinct channel functions and properties. Their activation during ischemic stroke can be beneficial, detrimental, or even both. In this review, we focus on discussing the interesting features of stroke-related TRP channels and summarizing the underlying cellular and molecular mechanisms responsible for their involvement in ischemic brain injury.

Post-traumatic stress disorder: the role of the amygdala and potential therapeutic interventions - a review

Introduction

Post-traumatic stress disorder (PTSD) is a psychiatric disorder triggered by exposure to a life-threatening or sexually violent traumatic event, and is characterized by symptoms involving intrusive re-experiencing, persistent avoidance of associated stimuli, emotional and cognitive disturbances, and hyperarousal for long periods after the trauma has occurred. These debilitating symptoms induce occupational and social impairments that contribute to a significant clinical burden for PTSD patients, and substantial socioeconomic costs, reaching approximately $20,000 dollars per individual with PTSD each year in the US. Despite increased translational research focus in the field of PTSD, the development of novel, effective pharmacotherapies for its treatment remains an important unmet clinical need.

Observations

In this review, we summarize the evidence implicating dysfunctional activity of the amygdala in the pathophysiology of PTSD. We identify the transient receptor potential canonical (TRPC) ion channels as promising drug targets given their distribution in the amygdala, and evidence from animal studies demonstrating their role in fear response modulation. We discuss the evidence-based pharmacotherapy and psychotherapy treatment approaches for PTSD.

Discussion

In view of the prevalence and economic burden associated with PTSD, further investigation is warranted into novel treatment approaches based on our knowledge of the involvement of brain circuitry and the role of the amygdala in PTSD, as well as the potential added value of combined pharmacotherapy and psychotherapy to better manage PTSD symptoms.

Safety, Tolerability, and Pharmacokinetics of Oral BI 1358894 in Healthy Japanese Male Volunteers

BACKGROUND AND OBJECTIVES: BI 1358894, a novel small-molecule inhibitor of transient receptor potential canonical ion channels, is under development for treatment of major depressive disorder. Phase I trials assessing the safety and pharmacokinetics of BI 1358894 in Caucasian male healthy volunteers (HVs) have been performed. This Phase I, double-blind, placebo-controlled, parallel-group trial assessed the safety, tolerability and pharmacokinetics of BI 1358894 in Japanese male HVs.

METHODS

Male HVs were randomized to receive oral BI 1358894 (n = 18) or placebo (n = 6) after a high-fat, high-calorie meal within three dose groups (50 mg, 100 mg, 200 mg), administered sequentially in dose-ascending order. The primary endpoint was number of HVs with drug-related adverse events (DRAEs). Secondary endpoints were the pharmacokinetic parameters of BI 1358894.

RESULTS

Overall, 24 male HVs entered the trial [mean (standard deviation) age: 30.0 (7.6) years]. DRAEs occurred in 3/18 HVs (BI 1358894 100 mg group: one HV experienced dizziness and headache; BI 1358894 200 mg group: one HV experienced headache, another reported sleep disorder). BI 1358894 exposure increased dose dependently and proportionally, peaking 4-6 h after administration before declining in a multiphasic manner with a terminal elimination half-life of ~70 h in the 50 mg and 100 mg dose groups, and 203 h in the 200 mg dose group.

CONCLUSION

BI 1358894 was well tolerated with a favorable pharmacokinetic profile in Japanese male HVs, similar to findings from a previous study in Caucasian male HVs.

TRIAL REGISTRATION

ClinicalTrials.gov (NCT03875001; 08-Mar-2019).

TRPC4 deletion elicits behavioral defects in sociability by dysregulating expression of microRNA-138-2

To investigate whether the defects in transient receptor potential canonical 4 (TRPC4), which is strongly expressed in the hippocampus, are implicated in ASD, we examined the social behaviors of mice in which Trpc4 was deleted (Trpc4-/-). Trpc4-/- mice displayed the core symptoms of ASD, namely, social disability and repetitive behaviors. In microarray analysis of the hippocampus, microRNA (miR)-138-2, the precursor of miR-138, was upregulated in Trpc4-/- mice. We also found that binding of Matrin3 (MATR3), a selective miR-138-2 binding nuclear protein, to miR-138-2 was prominently enhanced, resulting in the downregulation of miR-138 in Trpc4-/- mice. Some parameters of the social defects and repetitive behaviors in the Trpc4-/- mice were rescued by increased miR-138 levels following miR-138-2 infusion in the hippocampus. Together, these results suggest that Trpc4 regulates some signaling components that oppose the development of social behavioral deficits through miR-138 and provide a potential therapeutic strategy for ASD.

Molecular mechanisms of cholinergic neurotransmission in visceral smooth muscles with a focus on receptor-operated TRPC4 channel and impairment of gastrointestinal motility by general anaesthetics and anxiolytics

Acetylcholine is the primary excitatory neurotransmitter in visceral smooth muscles, wherein it binds to and activates two muscarinic receptors subtypes, M2 and M3, thus causing smooth muscle excitation and contraction. The first part of this review focuses on the types of cells involved in cholinergic neurotransmission and on the molecular mechanisms underlying acetylcholine-induced membrane depolarisation, which is the central event of excitation-contraction coupling causing Ca2+ entry via L-type Ca2+ channels and smooth muscle contraction. Studies of the muscarinic cation current in intestinal myocytes (mICAT) revealed its main molecular counterpart, receptor-operated TRPC4 channel, which is activated in synergy by both M2 and M3 receptors. M3 receptors activation is of permissive nature, while activation of M2 receptors via Gi/o proteins that are coupled to them plays a direct role in TRPC4 opening. Our understanding of signalling pathways underlying mICAT generation has vastly expanded in recent years through studies of TRPC4 gating in native cells and its regulation in heterologous cells. Recent studies using muscarinic receptor knockout have established that at low agonist concentration activation of both M2 receptor and the M2/M3 receptor complex elicits smooth muscle contraction, while at high agonist concentration M3 receptor function becomes dominant. Based on this knowledge, in the second part of this review we discuss the cellular and molecular mechanisms underlying the numerous anticholinergic effects on neuroactive drugs, in particular general anaesthetics and anxiolytics, which can significantly impair gastrointestinal motility. This article is part of the Special Issue on "Ukrainian Neuroscience".

Quantitative trait loci mapping of innate fear behavior in day-old F2 chickens of Japanese Oh-Shamo and White Leghorn breeds using restriction site-associated DNA sequencing

Understanding the genetic mechanisms that underlie innate fear behavior is essential for improving the management and performance of the poultry industry. This study aimed to map QTL associated with innate fear responses in open field (OF) and tonic immobility (TI) tests, using an F2 chicken intercross population between 2 behaviorally distinct breeds: the aggressive Japanese Oh-Shamo (OSM) and the docile White Leghorn T-line (WL-T). Genome-wide QTL analysis for the OF and TI traits was conducted using 2,109 single nucleotide polymorphism (SNP) markers obtained through restriction site-associated DNA sequencing (RAD-seq). While several suggestive QTL were identified for TI and OF traits at genome-wide 20% significance threshold levels, the analysis revealed 2 significant QTL for 2 OF traits (total distance and maximum speed) at genome-wide 5% significance threshold levels. These significant QTL were located between 12.34 and 30.49 megabase (Mb) on chromosome 1 and between 40.02 and 63.38 Mb on chromosome 2, explaining 6.75 to 7.40% of the total variances. These findings provide valuable insights for the poultry industry, particularly in refining chicken management strategies and informing targeted breeding efforts.

Effects of Single and Multiple Ascending Doses of BI 1358894 in Healthy Male Volunteers on Safety, Tolerability and Pharmacokinetics: Two Phase I Partially Randomised Studies

INTRODUCTION

The transient receptor potential canonical (TRPC) ion channels have been implicated in the pathophysiology of major depressive disorder (MDD), and TRPC inhibition has been shown to reduce depressive-like behaviour in rodent models of depression. BI 1358894, a small-molecule inhibitor of TRPC ion channels, is currently being developed for the treatment of MDD.

OBJECTIVE

Two phase I studies assessed the safety, tolerability, and pharmacokinetics (PK) of oral BI 1358894 in fed and fasted states following a single ascending dose (SAD) [NCT03210272/1402-0001] and multiple ascending doses (MAD) [NCT03754959/1402-0002] in healthy male volunteers. In addition, any potential food effect was evaluated after a single dose.

METHODS

In both studies, eligible healthy male volunteers (aged 18-45 years; body mass index of 18.5-29.9 kg/m2) were allocated to receive BI 1358894 or placebo. In the SAD study (1402-0001), volunteers were randomised 3:1 to receive BI 1358894 or placebo in fasted (3, 6, 10, 25, 50, 100, or 200 mg) and fed states (200 mg). The food effect part was conducted as an open-label, randomised, two-way crossover study at doses of 50 and 100 mg in fasted and fed states (high-calorie, high-fat breakfast). For the MAD study (1402-0002), volunteers were randomised 4:1 to receive BI 1358894 (10, 25, 50, 100, or 200 mg) or placebo once daily for 14 days under fed conditions. Primary endpoint (both studies): number of volunteers with drug-related adverse events (DRAEs). Secondary PK endpoints for study 1402-0001: area under the concentration-time curve (AUC) from time zero extrapolated to infinity (AUC∞), maximum plasma concentration (Cmax), and AUC from time zero to the last quantifiable data time point (AUC0-tz). Secondary PK endpoints for study 1402-0002: AUC over 0-24 h (AUC0-24), Cmax after the first dose, and steady-state AUC and Cmax over a uniform dosing interval (AUCτ,ss and Cmax,ss, respectively) after the last dose.

RESULTS

BI 1358894 was well tolerated at doses ≤ 200 mg under all tested conditions and no dose dependency was observed in DRAE frequency for either study. In the SAD study, BI 1358894 exposure increased dose proportionally across 3-50 mg in the fasted state and across 50-200 mg in the fed state. A positive food effect was observed at the tested doses. In the MAD study, BI 1358894 exposure increased less than dose proportionally across 10-200 mg.

CONCLUSIONS

These studies demonstrate that BI 1358894 is well tolerated in healthy male volunteers following single and multiple doses, with no dose dependency observed in DRAE frequency. BI 1358894 exposure increased dose dependently in both the SAD and MAD studies, with higher exposure of BI 1358894 observed in the fed state.

CLINICALTRIALS REGISTRATION

These trials have been registered on ClinicalTrials.gov: NCT03210272/1402-0001 (registered on 6 July 2017) and NCT03754959/1402-0002 (registered on 27 November 2018).

Effect of BI 1358894 on Cholecystokinin-Tetrapeptide (CCK-4)-Induced Anxiety, Panic Symptoms, and Stress Biomarkers: A Phase I Randomized Trial in Healthy Males

INTRODUCTION

Depression, anxiety, and/or panic disorder are often comorbid and have a complex etiology mediated through the same neuronal network. Cholecystokinin-tetrapeptide (CCK-4), a synthetic analog of the endogenous neuropeptide cholecystokinin (CCK), is thought to be implicated in this network. The CCK-4 challenge model is an accepted method of investigating the pathophysiology of panic and has been shown to mediate neuronal activation via the transient receptor potential canonical (TRPC) ion channels.

OBJECTIVES

This study aimed to assess the pharmacodynamic effects of BI 1358894, a small-molecule inhibitor of TRPC ion channel members 4 and 5 (TRPC4/5), on CCK-4-induced anxiety/panic-like symptoms and evaluate circuit engagement.

METHODS

Twenty healthy male CCK-4-sensitive volunteers entered a Phase I, double blind, randomized, two-way cross-over, single dose, placebo-controlled trial. Randomization was to oral BI 1358894 100 mg in the fed state followed by oral placebo in the fed state, or vice versa. Treatments were administered 5 h prior to intravenous CCK-4 50 µg. The primary endpoint was maximum change from baseline of the Panic Symptom Scale (PSS) sum intensity score after CCK-4 injection. Further endpoints included the emotional faces visual analog score (EVAS), the Spielberger State-Trait Anxiety Inventory (STAI), plasma adrenocorticotropic hormone (ACTH), and serum cortisol values. The safety and tolerability of BI 1358894 was assessed based on a number of parameters including occurrence of adverse events (AEs). All pharmacodynamic, pharmacokinetic, and safety endpoints were analyzed using descriptive statistics.

RESULTS

Single oral doses of BI 1358894 were generally well tolerated by the healthy male volunteers included in this study. Adjusted mean maximum change from baseline in PSS sum intensity score was 24.4 % lower in volunteers treated with BI 1358894 versus placebo, while adjusted mean maximum change from baseline of EVAS was reduced by 19.2 % (BI 1358894 vs placebo). The STAI total score before CCK-4 injection was similar in both groups (placebo: 25.1; BI 1358894: 24.3). Relative to placebo, BI 1358894 reduced CCK-4-induced mean maximum plasma ACTH and serum cortisol values by 58.6 % and 27.3 %, respectively. Investigator-assessed drug-related AEs were reported for 13/20 participants (65.0 %). There were no serious or severe AEs, AEs of special interest, AEs leading to discontinuation of trial medication, or deaths.

CONCLUSIONS

Overall, BI 1358894 reduced psychological and physiological responses to CCK-4 compared with placebo, as measured by PSS, subjective EVAS and objectively measured stress biomarkers. BI 1358894 had a positive safety profile, and single oral doses were well tolerated by the healthy volunteers. This trial (NCT03904576/1402-0005) was registered on Clinicaltrials.gov on 05.04.19.

Discovery of potential novel TRPC5 inhibitors by virtual screening and bioassay

The transient receptor potential canonical channel 5 (TRPC5), a member of the TRPC family, plays a crucial role in the regulation of various physiological activities and diseases, including those related to the central nervous system, cardiovascular system, kidney, and cancer. As a nonselective cation channel, TRPC5 mainly controls the influx of extracellular Ca2+ into cells, thereby modulating cellular depolarization and intracellular ion concentration. Inhibition of TRPC5 by small molecules presents a promising approach for the treatment of TRPC5-associated diseases. In this study, we conducted a comprehensive virtual screening of more than 1.5 million molecules from the Chemdiv database (https://www.chemdiv.com) to identify potential inhibitors of hTRPC5, utilizing the published structures and binding sites of hTRPC5 as a basis. Lipinski's rule, Veber's rule, PAINS filters, pharmacophore analysis, molecular docking, ADMET evaluation and cluster analysis methods were applied for the screening. From this rigorous screening process, 18 candidates exhibiting higher affinities to hTRPC5 were subsequently evaluated for their inhibitory effects on Ca2+ influx using a fluorescence-based assay. Notably, two molecules, namely SML-1 and SML-13, demonstrated significant inhibition of intracellular Ca2+ levels in hTRPC5-overexpressing HEK 293T cells, with IC50 values of 10.2 μM and 10.3 μM, respectively. These findings highlight SML-1 and SML-13 as potential lead molecules for the development of therapeutics targeting hTRPC5 and its associated physiological activities and diseases.

Targeting Nociceptive Neurons and Transient Receptor Potential Channels for the Treatment of Migraine

Migraine is a neurovascular disorder that affects approximately 12% of the global population. While its exact causes are still being studied, researchers believe that nociceptive neurons in the trigeminal ganglia play a key role in the pain signals of migraine. These nociceptive neurons innervate the intracranial meninges and convey pain signals from the meninges to the thalamus. Targeting nociceptive neurons is considered promising due to their accessibility and distinct molecular profile, which includes the expression of several transient receptor potential (TRP) channels. These channels have been linked to various pain conditions, including migraine. This review discusses the role and mechanisms of nociceptive neurons in migraine, the challenges of current anti-migraine drugs, and the evidence for well-studied and emerging TRP channels, particularly TRPC4, as novel targets for migraine prevention and treatment.

Inhibition of Canonical Transient Receptor Potential Channels 4/5 with Highly Selective and Potent Small-Molecule HC-070 Alleviates Mechanical Hypersensitivity in Rat Models of Visceral and Neuropathic Pain

Transient receptor potential channels C4/C5 are widely expressed in the pain pathway. Here, we studied the putative analgesic efficacy of the highly selective and potent TRPC4/C5 antagonist HC-070 in rats. Inhibitory potency on human TRPC4 was assessed by using the whole-cell manual patch-clamp technique. Visceral pain sensitivity was assessed by the colonic distension test after intra-colonic trinitrobenzene sulfonic acid injection and partial restraint stress. Mechanical pain sensitivity was assessed by the paw pressure test in the chronic constriction injury (CCI) neuropathic pain model. We confirm that HC-070 is a low nanomolar antagonist. Following single oral doses (3-30 mg/kg in male or female rats), colonic hypersensitivity was significantly and dose-dependently attenuated, even fully reversed to baseline. HC-070 also had a significant anti-hypersensitivity effect in the established phase of the CCI model. HC-070 did not have an effect on the mechanical withdrawal threshold of the non-injured paw, whereas the reference compound morphine significantly increased it. Analgesic effects are observed at unbound brain concentrations near the 50% inhibitory concentration (IC₅₀) recorded in vitro. This suggests that analgesic effects reported here are brought about by TRPC4/C5 blocking in vivo. The results strengthen the idea that TRPC4/C5 antagonism is a novel, safe non-opioid treatment for chronic pain.

Hypothalamic warm-sensitive neurons require TRPC4 channel for detecting internal warmth and regulating body temperature in mice

Precise monitoring of internal temperature is vital for thermal homeostasis in mammals. For decades, warm-sensitive neurons (WSNs) within the preoptic area (POA) were thought to sense internal warmth, using this information as feedback to regulate body temperature (T_core). However, the cellular and molecular mechanisms by which WSNs measure temperature remain largely undefined. Via a pilot genetic screen, we found that silencing the TRPC4 channel in mice substantially attenuated hypothermia induced by light-mediated heating of the POA. Loss-of-function studies of TRPC4 confirmed its role in warm sensing in GABAergic WSNs, causing additional defects in basal temperature setting, warm defense, and fever responses. Furthermore, TRPC4 antagonists and agonists bidirectionally regulated T_core. Thus, our data indicate that TRPC4 is essential for sensing internal warmth and that TRPC4-expressing GABAergic WSNs function as a novel cellular sensor for preventing T_core from exceeding set-point temperatures. TRPC4 may represent a potential therapeutic target for managing T_core.

The effects of transient receptor potential cation channel inhibition by BI 1358894 on cortico-limbic brain reactivity to negative emotional stimuli in major depressive disorder

Abnormal emotional processing in major depressive disorder (MDD) has been associated with increased activation to negative stimuli in cortico-limbic brain regions. The authors investigated whether treatment with BI 1358894, a small-molecule inhibitor of the transient receptor potential cation channel subfamily C leads to attenuated activity in these areas in MDD patients. 73 MDD patients were randomized to receive a single oral dose of BI 1358894 (100 mg), citalopram (20 mg), or matching placebo. Brain responses to emotional faces and scenes were investigated using functional magnetic resonance imaging. Primary endpoints were BOLD signal changes in response to negative faces in cortico-limbic brain regions, i.e. bilateral amygdala (AMY), dorsolateral prefrontal cortex, anterior insula (AI), and anterior cingulate cortex. Secondary endpoints were BOLD signal changes in response to negative scenes. For each region, separate ANOVA models were computed for the comparison of treatments (BI 1358894 or citalopram) vs. placebo. The adjusted treatment differences in the % BOLD signal changes in the faces task showed that BI 1358894 induced signal reduction in bilateral AMY and left AI. In the scenes task, BI 1358894 demonstrated significant signal reduction in bilateral AMY, AI, anterior cingulate cortex and left dorsolateral prefrontal cortex. Citalopram failed to induce any significant reductions in BOLD signal in both tasks. BI 1358894-mediated inhibition of the transient receptor potential cation channel subfamily resulted in strong signal reduction in cortico-limbic brain regions, thereby supporting development of this mechanism of action for MDD patients.

Ionic signalling mechanisms involved in neurokinin-3 receptor-mediated augmentation of fear-potentiated startle response in the basolateral amygdala

The tachykinin peptides include substance P (SP), neurokinin A and neurokinin B, which interact with three G-protein-coupled neurokinin receptors, NK1Rs, NK2Rs and NK3Rs, respectively. Whereas high densities of NK3Rs have been detected in the basolateral amygdala (BLA), the functions of NK3Rs in this brain region have not been determined. We found that activation of NK3Rs by application of the selective agonist, senktide, persistently excited BLA principal neurons. NK3R-elicited excitation of BLA neurons was mediated by activation of a non-selective cation channel and depression of the inwardly rectifying K+ (Kir) channels. With selective channel blockers and knockout mice, we further showed that NK3R activation excited BLA neurons by depressing the G protein-activated inwardly rectifying K+ (GIRK) channels and activating TRPC4 and TRPC5 channels. The effects of NK3Rs required the functions of phospholipase Cβ (PLCβ), but were independent of intracellular Ca2+ release and protein kinase C. PLCβ-mediated depletion of phosphatidylinositol 4,5-bisphosphate was involved in NK3R-induced excitation of BLA neurons. Microinjection of senktide into the BLA of rats augmented fear-potentiated startle (FPS) and this effect was blocked by prior injection of the selective NK3R antagonist SB 218795, suggesting that activation of NK3Rs in the BLA increased FPS. We further showed that TRPC4/5 and GIRK channels were involved in NK3R-elicited facilitation of FPS. Our results provide a cellular and molecular mechanism whereby NK3R activation excites BLA neurons and enhances FPS. KEY POINTS: Activation of NK3 receptors (NK3Rs) facilitates the excitability of principal neurons in rat basolateral amygdala (BLA). NK3R-induced excitation is mediated by inhibition of GIRK channels and activation of TRPC4/5 channels. Phospholipase Cβ and depletion of phosphatidylinositol 4,5-bisphosphate are necessary for NK3R-mediated excitation of BLA principal neurons. Activation of NK3Rs in the BLA facilitates fear-potentiated startle response. GIRK channels and TRPC4/5 channels are involved in NK3R-mediated augmentation of fear-potentiated startle.

Interactions between the Polysialylated Neural Cell Adhesion Molecule and the Transient Receptor Potential Canonical Channels 1, 4, and 5 Induce Entry of Ca2+ into Neurons

The neural cell adhesion molecule (NCAM) plays important functional roles in the developing and mature nervous systems. Here, we show that the transient receptor potential canonical (TRPC) ion channels TRPC1, -4, and -5 not only interact with the intracellular domains of the transmembrane isoforms NCAM140 and NCAM180, but also with the glycan polysialic acid (PSA) covalently attached to the NCAM protein backbone. NCAM antibody treatment leads to the opening of TRPC1, -4, and -5 hetero- or homomers at the plasma membrane and to the influx of Ca²⁺ into cultured cortical neurons and CHO cells expressing NCAM, PSA, and TRPC1 and -4 or TRPC1 and -5. NCAM-stimulated Ca²⁺ entry was blocked by the TRPC inhibitor Pico145 or the bacterial PSA homolog colominic acid. NCAM-stimulated Ca²⁺ influx was detectable neither in NCAM-deficient cortical neurons nor in TRPC1/4- or TRPC1/5-expressing CHO cells that express NCAM, but not PSA. NCAM-induced neurite outgrowth was reduced by TRPC inhibitors and a function-blocking TRPC1 antibody. A characteristic signaling feature was that extracellular signal-regulated kinase 1/2 phosphorylation was also reduced by TRPC inhibitors. Our findings indicate that the interaction of NCAM with TRPC1, -4, and -5 contributes to the NCAM-stimulated and PSA-dependent Ca²⁺ entry into neurons thereby influencing essential neural functions.

Spinal TRPA1 Contributes to the Mechanical Hypersensitivity Effect Induced by Netrin-1

Netrin-1, a chemoattractant expressed by floor plate cells, and one of its receptors (deleted in colorectal cancer) has been associated with pronociceptive actions in a number of pain conditions. Here, we addressed the question of whether spinal TRPC4/C5 or TRPA1 are among the downstream receptors contributing to pronociceptive actions induced by netrin-1. The experiments were performed on rats using a chronic intrathecal catheter for administration of netrin-1 and antagonists of TRPC4/C5 or TRPA1. Pain sensitivity was assessed behaviorally by using mechanical and heat stimuli. Effect on the discharge rate of rostral ventromedial medullary (RVM) pain control neurons was studied in lightly anesthetized animals. Netrin-1, in a dose-related fashion, induced mechanical hypersensitivity that lasted up to three weeks. Netrin-1 had no effect on heat nociception. Mechanical hypersensitivity induced by netrin-1 was attenuated by TRPA1 antagonist Chembridge-5861528 and by the control analgesic compound pregabalin both during the early (first two days) and late (third week) phase of hypersensitivity. TRPC4/C5 antagonist ML-204 had a weak antihypersensitivity effect that was only in the early phase, whereas TRPC4/C5 antagonist HC-070 had no effect on hypersensitivity induced by netrin-1. The discharge rate in pronociceptive ON-like RVM neurons was increased by netrin-1 during the late but not acute phase, whereas netrin-1 had no effect on the discharge rate of antinociceptive RVM OFF-like neurons. The results suggest that spinal TRPA1 receptors and pronociceptive RVM ON-like neurons are involved in the maintenance of submodality-selective pronociceptive actions induced by netrin-1 in the spinal cord.

TRPC4 and GIRK channels underlie neuronal coding of firing patterns that reflect Gq/11-Gi/o coincidence signals of variable strengths

SignificanceNeurons communicate by releasing neurotransmitters, many of which act at G protein-coupled receptors. Although it is well known that G_q/11 accelerates action potential firing while G_i/o inhibits firing, how firing patterns change in response to simultaneous activation of G_q/11 and G_i/o remains elusive, especially because the relative strength of G_q/11 versus G_i/o activation varies greatly from event to event. This study reveals that neurons encode distinct messages that reflect coincident G_q/11 and G_i/o stimulation by activating two ion channels, TRPC4 and GIRK. The resulting firing patterns, composed of burst, pause, and firing recovery phases, reflect both the occurrence of coincident G_q/11 and G_i/o activation and their relative strengths. With these, we may begin to interpret the language of neurons.

New Insights on Gene by Environmental Effects of Drugs of Abuse in Animal Models Using GeneNetwork

Gene-by-environment interactions are important for all facets of biology, especially behaviour. Families of isogenic strains of mice, such as the BXD strains, are excellently placed to study these interactions, as the same genome can be tested in multiple environments. BXD strains are recombinant inbred mouse strains derived from crossing two inbred strains—C57BL/6J and DBA/2J mice. Many reproducible genometypes can be leveraged, and old data can be reanalysed with new tools to produce novel insights. We obtained drug and behavioural phenotypes from Philip et al. Genes, Brain and Behaviour 2010, and reanalysed their data with new genotypes from sequencing, as well as new models (Genome-wide Efficient Mixed Model Association (GEMMA) and R/qtl2). We discovered QTLs on chromosomes 3, 5, 9, 11, and 14, not found in the original study. We reduced the candidate genes based on their ability to alter gene expression or protein function. Candidate genes included Slitrk6 and Cdk14. Slitrk6, in a Chromosome14 QTL for locomotion, was found to be part of a co-expression network involved in voluntary movement and associated with neuropsychiatric phenotypes. Cdk14, one of only three genes in a Chromosome5 QTL, is associated with handling induced convulsions after ethanol treatment, that is regulated by the anticonvulsant drug valproic acid. By using families of isogenic strains, we can reanalyse data to discover novel candidate genes involved in response to drugs of abuse.

Distribution and Assembly of TRP Ion Channels

In the last several decades, a large family of ion channels have been identified and studied intensively as cellular sensors for diverse physical and/or chemical stimuli. Named transient receptor potential (TRP) channels, they play critical roles in various aspects of cellular physiology. A large number of human hereditary diseases are found to be linked to TRP channel mutations, and their dysregulations lead to acute or chronical health problems. As TRP channels are named and categorized mostly based on sequence homology rather than functional similarities, they exhibit substantial functional diversity. Rapid advances in TRP channel study have been made in recent years and reported in a vast body of literature; a summary of the latest advancements becomes necessary. This chapter offers an overview of current understandings of TRP channel distribution and subunit assembly.

Depression-like behavior associated with E/I imbalance of mPFC and amygdala without TRPC channels in mice of knockout IL-10 from microglia

Depression has a growing impact on public health. Accumulating evidence supports an association between depression and increased immune system activity. IL-10 is a key cytokine that inhibits excessive inflammatory responses and is related to the anti-inflammatory and protective functions of the central nervous system (CNS). Cx3cr1^CreERIL-10^-/- mice were used in our study. We aimed to identify the role of IL-10 in microglia in depression and anxiety-like behavior. We performed a series of behavioral tests on the mice; the Cx3cr1^CreERIL-10^-/- male mice showed depression- and anxiety-like behavior compared with the littermates. The expression of transient receptor potential canonical 5 (TRPC5) decreased in both the medial prefrontal cortex (mPFC) and amygdala regions. The cytokines IL-1β and IL-6 increased, and IL-10 was decreased by western blotting. The knockout mice showed different trends in the effects of synaptic proteins. In the mPFC, IL-10 knockout induced a decrease in NR2B and synaptophysin; in the amygdala region, there was a significant increase in NR2B and PSD95. IL-10 knockout from microglia induced a decrease in GAD67 and parvalbumin (Pv) in the mPFC, but not in the amygdala. Our results showed enhanced depression and anxiety-like behavior in the Cx3cr1^CreER IL-10^-/- mice, which could be related to an imbalance in local excitatory and inhibitory transmission, as well as neuroinflammation in the mPFC and amygdala. This imbalance was associated with increased local inflammation. Although many studies have demonstrated the role of TRPC channels in emotional responses, our study showed that TRPC was not involved in this process in Cx3cr1^CreERIL-10^-/- mice.

Advances in TRP channel drug discovery: from target validation to clinical studies

Transient receptor potential (TRP) channels are multifunctional signalling molecules with many roles in sensory perception and cellular physiology. Therefore, it is not surprising that TRP channels have been implicated in numerous diseases, including hereditary disorders caused by defects in genes encoding TRP channels (TRP channelopathies). Most TRP channels are located at the cell surface, which makes them generally accessible drug targets. Early drug discovery efforts to target TRP channels focused on pain, but as our knowledge of TRP channels and their role in health and disease has grown, these efforts have expanded into new clinical indications, ranging from respiratory disorders through neurological and psychiatric diseases to diabetes and cancer. In this Review, we discuss recent findings in TRP channel structural biology that can affect both drug development and clinical indications. We also discuss the clinical promise of novel TRP channel modulators, aimed at both established and emerging targets. Last, we address the challenges that these compounds may face in clinical practice, including the need for carefully targeted approaches to minimize potential side-effects due to the multifunctional roles of TRP channels.

TRP channels in health and disease at a glance

The transient receptor potential (TRP) channel superfamily consists of a large group of non-selective cation channels that serve as cellular sensors for a wide spectrum of physical and environmental stimuli. The 28 mammalian TRPs, categorized into six subfamilies, including TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPA (ankyrin), TRPML (mucolipin) and TRPP (polycystin), are widely expressed in different cells and tissues. TRPs exhibit a variety of unique features that not only distinguish them from other superfamilies of ion channels, but also confer diverse physiological functions. Located at the plasma membrane or in the membranes of intracellular organelles, TRPs are the cellular safeguards that sense various cell stresses and environmental stimuli and translate this information into responses at the organismal level. Loss- or gain-of-function mutations of TRPs cause inherited diseases and pathologies in different physiological systems, whereas up- or down-regulation of TRPs is associated with acquired human disorders. In this Cell Science at a Glance article and the accompanying poster, we briefly summarize the history of the discovery of TRPs, their unique features, recent advances in the understanding of TRP activation mechanisms, the structural basis of TRP Ca2+ selectivity and ligand binding, as well as potential roles in mammalian physiology and pathology.

Involvement of TRPC5 channels, inwardly rectifying K+ channels, PLCβ and PIP2 in vasopressin-mediated excitation of medial central amygdala neurons

KEY POINTS

Activation of V_1a vasopressin receptors facilitates neuronal excitability in the medial nucleus of central amygdala (CeM) V_1a receptor activation excites about 80% CeM neurons by opening a cationic conductance and about 20% CeM neurons by suppressing an inwardly rectifying K⁺ (Kir) channel The cationic conductance activated by V_1a receptors is identified as TRPC5 channels PLCβ-mediated depletion of PIP₂ is involved in V_1a receptor-elicited excitation of CeM neurons Intracellular Ca²⁺ release and PKC are unnecessary for V_1a receptor-mediated excitation of CeM neurons ABSTRACT: Arginine vasopressin (AVP) serves as a hormone in the periphery to modulate water homeostasis and a neuromodulator in the brain to regulate a diverse range of functions including anxiety, social behaviour, cognitive activities and nociception. The amygdala is an essential brain region involved in modulating defensive and appetitive behaviours, pain and alcohol use disorders. Whereas activation of V_1a receptors in the medial nucleus of the central amygdala (CeM) increases neuronal excitability, the involved ionic and signalling mechanisms have not been determined. We found that activation of V_1a receptors in the CeM facilitated neuronal excitability predominantly by opening TRPC5 channels, although AVP excited about one fifth of the CeM neurons via suppressing an inwardly rectifying K⁺ (Kir) channel. G proteins and phospholipase Cβ (PLCβ) were required for AVP-elicited excitation of CeM neurons, whereas intracellular Ca²⁺ release and the activity of protein kinase C were unnecessary. Prevention of the depletion of phosphatidylinositol 4,5-bisphosphate (PIP₂ ) blocked AVP-induced excitation of CeM neurons, suggesting that PLCβ-mediated depletion of PIP₂ is involved in AVP-mediated excitation of CeM neurons. Our results may provide a cellular and molecular mechanism to explain the anxiogenic effects of AVP in the amygdala.

Canonical transient receptor potential channels and their modulators: biology, pharmacology and therapeutic potentials

Canonical transient receptor potential channels (TRPCs) are nonselective, high calcium permeability cationic channels. The TRPCs family includes TRPC1, TRPC2, TRPC3, TRPC4, TRPC5, TRPC6, and TRPC7. These channels are widely expressed in the cardiovascular and nervous systems and exist in many other human tissues and cell types, playing several crucial roles in the human physiological and pathological processes. Hence, the emergence of TRPCs modulators can help investigate these channels’ applications in health and disease. It is worth noting that the TRPCs subfamilies have structural and functional similarities, which presents a significant difficulty in screening and discovering of TRPCs modulators. In the past few years, only a limited number of selective modulators of TRPCs were detected; thus, additional research on more potent and more selective TRPCs modulators is needed. The present review focuses on the striking desired therapeutic effects of TRPCs modulators, which provides intel on the structural modification of TRPCs modulators and further pharmacological research. Importantly, TRPCs modulators can significantly facilitate future studies of TRPCs and TRPCs related diseases.

Role of Oxidative Stress and Ca2+ Signaling in Psychiatric Disorders

Psychiatric disorders are caused by complex and diverse factors, and numerous mechanisms have been proposed for the pathogenesis of these disorders. Accumulating evidence suggests that oxidative stress is one of the general factors involved in the pathogenesis/pathophysiology of major psychiatric disorders, including bipolar disorder, depression, anxiety disorder, and schizophrenia. Indeed, some clinical trials have shown improvement of the symptoms of these disorders by antioxidant supplementation. However, the molecular basis for the relationship between oxidative stress and the pathogenesis of psychiatric disorders remains largely unknown. In general, Ca2+ channels play central roles in neuronal functions, including neuronal excitability, neurotransmitter release, synaptic plasticity, and gene regulation, and genes that encode Ca2+ channels have been found to be associated with psychiatric disorders. Notably, a class of Ca2+-permeable transient receptor potential (TRP) cation channels is activated by changes in cellular redox status, whereby these TRP channels can link oxidative stress to Ca2+ signals. Given the unique characteristic of redox-sensitive TRP channels, these channels could be a target for delineating the pathogenesis or pathophysiology of psychiatric disorders. In this review, we summarize the outcomes of clinical trials for antioxidant treatment in patients with psychiatric disorders and the current insights into the physiological/pathological significance of redox-sensitive TRP channels in the light of neural functions, including behavioral phenotypes, and discuss the potential role of TRP channels in the pathogenesis of psychiatric disorders. Investigation of redox-sensitive TRP channels may lead to the development of novel therapeutic strategies for the treatment of psychiatric disorders.

Contribution of TRPC Channels in Neuronal Excitotoxicity Associated With Neurodegenerative Disease and Ischemic Stroke

The seven canonical members of transient receptor potential (TRPC) proteins form cation channels that evoke membrane depolarization and intracellular calcium concentration ([Ca²⁺] _i ) rise, which are not only important for regulating cell function but their deregulation can also lead to cell damage. Recent studies have implicated complex roles of TRPC channels in neurodegenerative diseases including ischemic stroke. Brain ischemia reduces oxygen and glucose supply to neurons, i.e., Oxygen and Glucose Deprivation (OGD), resulting in [Ca²⁺] _i elevation, ion dyshomeostasis, and excitotoxicity, which are also common in many forms of neurodegenerative diseases. Although ionotropic glutamate receptors, e.g., N-methyl-D-aspartate receptors, are well established to play roles in excitotoxicity, the contribution of metabotropic glutamate receptors and their downstream effectors, i.e., TRPC channels, should not be neglected. Here, we summarize the current findings about contributions of TRPC channels in neurodegenerative diseases, with a focus on OGD-induced neuronal death and rodent models of cerebral ischemia/reperfusion. TRPC channels play both detrimental and protective roles to neurodegeneration depending on the TRPC subtype and specific pathological conditions involved. When illustrated the mechanisms by which TRPC channels are involved in neuronal survival or death seem differ greatly, implicating diverse and complex regulation. We provide our own data showing that TRPC1/C4/C5, especially TRPC4, may be generally detrimental in OGD and cerebral ischemia/reperfusion. We propose that although TRPC channels significantly contribute to ischemic neuronal death, detailed mechanisms and specific roles of TRPC subtypes in brain injury at different stages of ischemia/reperfusion and in different brain regions need to be carefully and systematically investigated.

Discovery of novel modulators targeting human TRPC5: Docking-based virtual screening, molecular dynamics simulation and binding affinity predication

Canonical transient receptor potential channel 5 (TRPC5) plays a key role in the regulation of central nervous system, cardiovascular system, kidney disease, cancer, and could be also involved in liver function, arthritis, diabetes-associated complications and so on. However, evidence of TRPC5 function on cellular or organismic levels is sparse. There is still a need for identifying novel and efficient TRPC5 channel modulators to study TRPC5 function. In this study, based on the hTRPC5 structure obtained by homology modeling and the predicted binding site, we have performed virtual screening of 212,736 compounds from the specs database（http://www.specs.net) to find potential hTRPC5 modulators. Lipinski and Veber rules, ADMET (Absorption, Distribution, Metabolism, Excretion, Toxicity) and PAINS (Pan Assay Interference structures) filters were used to screen the large database. Further, multi-software combination docking, cluster analysis and interaction analysis were used to select 20 potential active candidates with novel skeleton. 4 Hits, bearing appreciable binding affinity with hTRPC5 were selected for 40ns all-atom molecular dynamics (MD) simulations under explicit water conditions. The MD simulation results suggested that the 4 Hits binding induces a slight structural change and stabilizes the hTRPC5 structure. In addition, decomposition free energy demonstrated that residues TRP434, LEU437, MET438, ALA441, ILE484, ILE487, LEU488, LEU491, LEU515, ILE517, LEU518, LEU521, PHE531, THR607, VAL610, ILE611, VAL615 played the critical role on system stability. 4 Hits, as potential modulators of hTRPC5, may be potential leads to develop effective therapeutics hTRPC5-associated diseases.

TRPC4 as a coincident detector of Gi/o and Gq/11 signaling: mechanisms and pathophysiological implications

TRPC channels are Ca²⁺-permeable nonselective cation channels activated downstream from phospholipase C (PLC). Although most TRPC channels can be activated by stimulating G_q/11-coupled receptors, TRPC4 requires simultaneous stimulation of G_i/o-coupled receptors, making it a perfect detector of coincident G_i/o and G_q/11 signaling. Evidence shows that activated Gα_i/o proteins work together with PLCδ1 to induce robust TRPC4 activation and the process is accelerated by stimulation of other PLC isozymes, such as PLCβ through G_q/11 proteins. Mechanistically, G_q/11-PLCβ activation produces triggering proton and calcium signals to initiate self-propagating PLCδ1 activity, crucial for G_i/o-mediated TRPC4 function. Thus, TRPC4-containing channels are activated under conditions not only when coincident G_i/o and G_q/11 stimulation occurs, but also when G_i/o stimulation coincides with proton and Ca²⁺ signals. The resulting cytosolic Ca²⁺ rise and membrane depolarization switch the inhibitory G_i/o response to excitation. The conditions and implications of G_i/o-mediated TRPC4 activation in physiology and pathophysiology warrant further investigation.

TRPC1/4/5 channels contribute to morphine-induced analgesic tolerance and hyperalgesia by enhancing spinal synaptic potentiation and structural plasticity

Opioid analgesics remain the mainstay for managing intractable chronic pain, but their use is limited by detrimental side effects such as analgesic tolerance and hyperalgesia. Calcium-dependent synaptic plasticity is a key determinant in opiates tolerance and hyperalgesia. However, the exact substrates for this calcium-dependent synaptic plasticity in mediating these maladaptive processes are largely unknown. Canonical transient receptor potential 1, 4, and 5 (TRPC1, 4, 5) proteins assemble into heteromultimeric nonselective cation channels with high Ca2+ permeability and influence various neuronal functions. However, whether and how TRPC1/4/5 channels contribute to the development of opiates tolerance and hyperalgesia remains elusive. Here, we show that TRPC1/4/5 channels contribute to the generation of morphine tolerance and hyperalgesia. Chronic morphine exposure leads to upregulation of TRPC1/4/5 channels in the spinal cord. Spinally expressed TRPC1, TPRC4, and TRPC5 are required for chronic morphine-induced synaptic long-term potentiation (LTP) as well as remodeling of synaptic spines in the dorsal horn, thereby orchestrating functional and structural plasticity during the course of morphine-induced hyperalgesia and tolerance. These effects are attributed to TRPC1/4/5-mediated Ca2+ elevation in the spinal dorsal horn induced by chronic morphine treatment. This study identifies TRPC1/4/5 channels as a promising novel target to prevent the unwanted morphine tolerance and hyperalgesia.

Intracellular acidification facilitates receptor-operated TRPC4 activation through PLCδ1 in a Ca2+ -dependent manner

KEY POINTS

Receptor-operated activation of TRPC4 cation channels requires G_i/o proteins and phospholipase-Cδ1 (PLCδ1) activation by intracellular Ca²⁺ . Concurrent stimulation of the G_q/11 pathway accelerates G_i/o activation of TRPC4, which is not mimicked by increasing cytosolic Ca²⁺ . The kinetic effect of G_q/11 was diminished by alkaline intracellular pH (pH_i ) and increased pH_i buffer capacity. Acidic pH_i (6.75-6.25) together with the cytosolic Ca²⁺ rise accelerated G_i/o -mediated TRPC4 activation. Protons exert their facilitation effect through Ca²⁺ -dependent activation of PLCδ1. The data suggest that the G_q/11 -PLCβ pathway facilitates G_i/o activation of TRPC4 through hydrolysing phosphatidylinositol 4,5-bisphosphate (PIP₂ ) to produce the initial proton signal that triggers a self-propagating PLCδ1 activity supported by regenerative H⁺ and Ca²⁺ . The findings provide novel mechanistic insights into receptor-operated TRPC4 activation by coincident G_q/11 and G_i/o pathways and shed light on how aberrant activation of TRPC4 may occur under pathological conditions to cause cell damage.

ABSTRACT

Transient Receptor Potential Canonical 4 (TRPC4) forms non-selective cation channels activated downstream from receptors that signal through G proteins. Our recent work suggests that TRPC4 channels are particularly coupled to pertussis toxin-sensitive G_i/o proteins, with a co-dependence on phospholipase-Cδ1 (PLCδ1). The G_i/o -mediated TRPC4 activation is dually dependent on and bimodally regulated by phosphatidylinositol 4,5-bisphosphate (PIP₂ ), the substrate hydrolysed by PLC, and intracellular Ca²⁺ . As a byproduct of PLC-mediated PIP₂ hydrolysis, protons have been shown to play an important role in the activation of Drosophila TRP channels. However, how intracellular pH affects mammalian TRPC channels remains obscure. Here, using patch-clamp recordings of HEK293 cells heterologously co-expressing mouse TRPC4β and the G_i/o -coupled μ opioid receptor, we investigated the role of intracellular protons on G_i/o -mediated TRPC4 activation. We found that acidic cytosolic pH greatly accelerated the rate of TRPC4 activation without altering the maximal current density and this effect was dependent on intracellular Ca²⁺ elevation. However, protons did not accelerate channel activation by directly acting upon TRPC4. We additionally demonstrated that protons exert their effect through sensitization of PLCδ1 to Ca²⁺ , which in turn promotes PLCδ1 activity and further potentiates TRPC4 via a positive feedback mechanism. The mechanism elucidated here helps explain how G_i/o and G_q/11 co-stimulation induces a faster activation of TRPC4 than G_i/o activation alone and highlights again the critical role of PLCδ1 in TRPC4 gating.

Evidence of a Role for the TRPC Subfamily in Mediating Oxidative Stress in Parkinson's Disease

Parkinson's disease (PD) represents one of the most common multifactorial neurodegenerative disorders affecting the elderly population. It is associated with the aggregation of α-synuclein protein and the loss of dopaminergic neurons in the substantia nigra pars compacta of the brain. The disease is mainly represented by motor symptoms, such as resting tremors, postural instability, rigidity, and bradykinesia, that develop slowly over time. Parkinson's disease can also manifest as disturbances in non-motor functions. Although the pathology of PD has not yet been fully understood, it has been suggested that the disruption of the cellular redox status may contribute to cellular oxidative stress and, thus, to cell death. The generation of reactive oxygen species and reactive nitrogen intermediates, as well as the dysfunction of dopamine metabolism, play important roles in the degeneration of dopaminergic neurons. In this context, the transient receptor potential channel canonical (TRPC) sub-family plays an important role in neuronal degeneration. Additionally, PD gene products, including DJ-1, SNCA, UCH-L1, PINK-1, and Parkin, also interfere with mitochondrial function leading to reactive oxygen species production and dopaminergic neuronal vulnerability to oxidative stress. Herein, we discuss the interplay between these various biochemical and molecular events that ultimately lead to dopaminergic signaling disruption, highlighting the recently identified roles of TRPC in PD.

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.

Sensory Neuron-Expressed TRPC4 Is a Target for the Relief of Psoriasiform Itch and Skin Inflammation in Mice

Psoriasis is an inflammatory skin disease associated with itch, which is a troublesome symptom with a few therapeutic options. TRPC4 is highly expressed in dorsal root ganglia (DRGs). Recently, we have revealed itch signaling in DRG neurons by which TRPC4 mediates itch to serotonergic antidepressants and demonstrated the antipruritic effect of the TRPC4 inhibitor ML204. However, the role of TRPC4 in acute and chronic itch is still largely unknown. Here, we have characterized the expression of TRPC4 in peptidergic DRG neurons and showed that acute itch induced by serotonin and histamine was attenuated in Trpc4-knockout mice and ML204-treated mice. We have also shown that silencing TRPC4 in DRG and its inhibition by intradermal injections were also effective in decreasing psoriatic itch after the repeated application of imiquimod, which is a preclinical model of psoriasis. Of clinical relevance, intradermal injections of ML204 in psoriasiform skin significantly reversed imiquimod-established chronic itch and cutaneous inflammation. Given that TRPC4 is expressed in human DRGs and a specific inhibitor is in clinical trials, our data not only expand our understanding of itch and psoriasis, but also reveal TRPC4 as a potential therapeutic target with considerable translational benefits.

TRPC1 Regulates the Activity of a Voltage-Dependent Nonselective Cation Current in Hippocampal CA1 Neurons

The cation channel subunit TRPC1 is strongly expressed in central neurons including neurons in the CA1 region of the hippocampus where it forms complexes with TRPC4 and TRPC5. To investigate the functional role of TRPC1 in these neurons and in channel function, we compared current responses to group I metabotropic glutamate receptor (mGluR I) activation and looked for major differences in dendritic morphology in neurons from TRPC1^+/+ and TRPC1^-/- mice. mGluR I stimulation resulted in the activation of a voltage-dependent nonselective cation current in both genotypes. Deletion of TRPC1 resulted in a modification of the shape of the current-voltage relationship, leading to an inward current increase. In current clamp recordings, the percentage of neurons that responded to depolarization in the presence of an mGluR I agonist with a plateau potential was increased in TRPC1^-/- mice. There was also a small increase in the minor population of CA1 neurons that have more than one apical dendrite in TRPC1^-/- mice. We conclude that TRPC1 has an inhibitory effect on receptor-operated nonselective cation channels in hippocampal CA1 neurons probably as a result of heterotetramer formation with other TRPC isoforms, and that TRPC1 deletion has only minor effects on dendritic morphology.

TRPC1 mediates slow excitatory synaptic transmission in hippocampal oriens/alveus interneurons

Hippocampal GABAergic interneurons play key roles in regulating principal cell activity and plasticity. Interneurons located in stratum oriens/alveus (O/A INs) receive excitatory inputs from CA1 pyramidal cells and express a Hebbian form of long-term potentiation (LTP) at their excitatory input synapses. This LTP requires the activation of metabotropic glutamate receptors 1a (mGluR1a) and Ca²⁺ entry via transient receptor potential (TRP) channels. However, the type of TRP channels involved in synaptic transmission at these synapses remains largely unknown. Using patch-clamp recordings, we show that slow excitatory postsynaptic currents (EPSCs) evoked in O/A INs are dependent on TRP channels but may be independent of phospholipase C. Using reverse transcription polymerase chain reaction (RT-PCR) we found that mRNA for TRPC 1, 3–7 was present in CA1 hippocampus. Using single-cell RT-PCR, we found expression of mRNA for TRPC 1, 4–7, but not TRPC3, in O/A INs. Using co-immunoprecipitation assays in HEK-293 cell expression system, we found that TRPC1 and TRPC4 interacted with mGluR1a. Co-immunoprecipitation in hippocampus showed that TRPC1 interacted with mGluR1a. Using immunofluorescence, we found that TRPC1 co-localized with mGluR1a in O/A IN dendrites, whereas TRPC4 localization appeared limited to O/A IN cell body. Down-regulation of TRPC1, but not TRPC4, expression in O/A INs using small interfering RNAs prevented slow EPSCs, suggesting that TRPC1 is an obligatory TRPC subunit for these EPSCs. Our findings uncover a functional role of TRPC1 in mGluR1a-mediated slow excitatory synaptic transmission onto O/A INs that could be involved in Hebbian LTP at these synapses.

TRPC channels: Structure, function, regulation and recent advances in small molecular probes

Transient receptor potential canonical (TRPC) channels constitute a group of receptor-operated calcium-permeable nonselective cation channels of the TRP superfamily. The seven mammalian TRPC members, which can be further divided into four subgroups (TRPC1, TRPC2, TRPC4/5, and TRPC3/6/7) based on their amino acid sequences and functional similarities, contribute to a broad spectrum of cellular functions and physiological roles. Studies have revealed complexity of their regulation involving several components of the phospholipase C pathway, G_i and G_o proteins, and internal Ca²⁺ stores. Recent advances in cryogenic electron microscopy have provided several high-resolution structures of TRPC channels. Growing evidence demonstrates the involvement of TRPC channels in diseases, particularly the link between genetic mutations of TRPC6 and familial focal segmental glomerulosclerosis. Because TRPCs were discovered by the molecular identity first, their pharmacology had lagged behind. This is rapidly changing in recent years owning to great efforts from both academia and industry. A number of potent tool compounds from both synthetic and natural products that selective target different subtypes of TRPC channels have been discovered, including some preclinical drug candidates. This review will cover recent advancements in the understanding of TRPC channel regulation, structure, and discovery of novel TRPC small molecular probes over the past few years, with the goal of facilitating drug discovery for the study of TRPCs and therapeutic development.

TRPC channels regulate Ca2+-signaling and short-term plasticity of fast glutamatergic synapses

Transient receptor potential (TRP) proteins form Ca²⁺-permeable, nonselective cation channels, but their role in neuronal Ca²⁺ homeostasis is elusive. In the present paper, we show that TRPC channels potently regulate synaptic plasticity by changing the presynaptic Ca²⁺-homeostasis of hippocampal neurons. Specifically, loss of TRPC1/C4/C5 channels decreases basal-evoked secretion, reduces the pool size of readily releasable vesicles, and accelerates synaptic depression during high-frequency stimulation (HFS). In contrast, primary TRPC5 channel-expressing neurons, identified by a novel TRPC5–τ-green fluorescent protein (τGFP) knockin mouse line, show strong short-term enhancement (STE) of synaptic signaling during HFS, indicating a key role of TRPC5 in short-term plasticity. Lentiviral expression of either TRPC1 or TRPC5 turns classic synaptic depression of wild-type neurons into STE, demonstrating that TRPCs are instrumental in regulating synaptic plasticity. Presynaptic Ca²⁺ imaging shows that TRPC activity strongly boosts synaptic Ca²⁺ dynamics, showing that TRPC channels provide an additional presynaptic Ca²⁺ entry pathway, which efficiently regulates synaptic strength and plasticity.

Transient receptor potential (TRP) proteins can form non-selective cation channels, but their role in synaptic transmission is poorly understood. This study shows that calcium-permeable TRPC channels provide an additional calcium entry pathway at presynaptic sites and are efficient regulators of synaptic strength and plasticity.

Cryo-EM structure of TRPC5 at 2.8-Å resolution reveals unique and conserved structural elements essential for channel function

The transient receptor potential canonical subfamily member 5 (TRPC5), one of seven mammalian TRPC members, is a nonselective calcium-permeant cation channel. TRPC5 is of considerable interest as a drug target in the treatment of progressive kidney disease, depression, and anxiety. Here, we present the 2.8-Å resolution cryo-electron microscopy (cryo-EM) structure of the mouse TRPC5 (mTRPC5) homotetramer. Comparison of the TRPC5 structure to previously determined structures of other TRPC and TRP channels reveals differences in the extracellular pore domain and in the length of the S3 helix. The disulfide bond at the extracellular side of the pore and a preceding small loop are essential elements for its proper function. This high-resolution structure of mTRPC5, combined with electrophysiology and mutagenesis, provides insight into the lipid modulation and gating mechanisms of the TRPC family of ion channels.

TRPC1 as a negative regulator for TRPC4 and TRPC5 channels

Transient receptor potential canonical (TRPC) channels are calcium permeable, non-selective cation channels with wide tissue-specific distribution. Among 7 TRPC channels, TRPC 1/4/5 and TRPC3/6/7 are subdivided based on amino acid sequence homology. TRPC4 and TRPC5 channels exhibit cationic current with homotetrameric form, but they also form heterotetrameric channel such as TRPC1/4 or TRPC1/5 once TRPC1 is incorporated. The expression of TRPC1 is ubiquitous whereas the expressions of TRPC4 and TRPC5 are rather focused in nervous system. With the help of conditional knock-out of TPRC1, 4 and/or 5 genes, TRPC channels made of these constituents are reported to be involved in various pathophysiological functions such as seizure, anxiety-like behaviour, fear, Huntington's disease, Parkinson's disease and many others. In heterologous expression system, many issues such as activation mechanism, stoichiometry and relative cation permeabilites of homomeric or heteromeric channels have been addressed. In this review, we discussed the role of TRPC1 channel per se in plasma membrane, role of TRPC1 in heterotetrameric conformation (TRPC1/4 or TRPC1/5) and relationship between TRPC1/4/5 channels, calcium influx and voltage-gated calcium channels.

TRPC channels are not required for graded persistent activity in entorhinal cortex neurons

Adaptive behavior requires the transient storage of information beyond the physical presence of external stimuli. This short-lasting form of memory involves sustained ("persistent") neuronal firing which may be generated by cell-autonomous biophysical properties of neurons or/and neural circuit dynamics. A number of studies from brain slices reports intrinsically generated persistent firing in cortical excitatory neurons following suprathreshold depolarization by intracellular current injection. In layer V (LV) neurons of the medial entorhinal cortex (mEC) persistent firing depends on the activation of cholinergic muscarinic receptors and is mediated by a calcium-activated nonselective cation current (I_CAN ). The molecular identity of this conductance remains, however, unknown. Recently, it has been suggested that the underlying ion channels belong to the canonical transient receptor potential (TRPC) channel family and include heterotetramers of TRPC1/5, TRPC1/4, and/or TRPC1/4/5 channels. While this suggestion was based on pharmacological experiments and on effects of TRP-interacting peptides, an unambiguous proof based on TRPC channel-depleted animals is pending. Here, we used two different lines of TRPC channel knockout mice, either lacking TRPC1-, TRPC4-, and TRPC5-containing channels or lacking all seven members of the TRPC family. We report unchanged persistent activity in mEC LV neurons in these animals, ruling out that muscarinic-dependent persistent activity depends on TRPC channels.

Review of Transient Receptor Potential Canonical (TRPC5) Channel Modulators and Diseases

Transient receptor potential canonical (TRPC) channels are highly homologous, nonselective cation channels that form many homo- and heterotetrameric channels. These channels are highly abundant in the brain and kidney and have been implicated in numerous diseases, such as depression, addiction, and chronic kidney disease, among others. Historically, there have been very few selective modulators of the TRPC family in order to fully understand their role in disease despite their physiological significance. However, that has changed recently and there has been a significant increase in interest in this family of channels which has led to the emergence of selective tool compounds, and even preclinical drug candidates, over the past few years. This review will cover these new advancements in the discovery of TRPC modulators and the emergence of newly reported structural information which will undoubtedly lead to even greater advancements.

Treasure troves of pharmacological tools to study transient receptor potential canonical 1/4/5 channels

Canonical or classical transient receptor potential 4 and 5 proteins (TRPC4 and TRPC5) assemble as homomers or heteromerize with TRPC1 protein to form functional nonselective cationic channels with high calcium permeability. These channel complexes, TRPC1/4/5, are widely expressed in nervous and cardiovascular systems, also in other human tissues and cell types. It is debatable that TRPC1 protein is able to form a functional ion channel on its own. A recent explosion of molecular information about TRPC1/4/5 has emerged including knowledge of their distribution, function, and regulation suggesting these three members of the TRPC subfamily of TRP channels play crucial roles in human physiology and pathology. Therefore, these ion channels represent potential drug targets for cancer, epilepsy, anxiety, pain, and cardiac remodelling. In recent years, a number of highly selective small-molecule modulators of TRPC1/4/5 channels have been identified as being potent with improved pharmacological properties. This review will focus on recent remarkable small-molecule agonists: (-)-englerin A and tonantzitlolone and antagonists: Pico145 and HC7090, of TPRC1/4/5 channels. In addition, this work highlights other recently identified modulators of these channels such as the benzothiadiazine derivative, riluzole, ML204, clemizole, and AC1903. Together, these treasure troves of agonists and antagonists of TRPC1/4/5 channels provide valuable hints to comprehend the functional importance of these ion channels in native cells and in vivo animal models. Importantly, human diseases and disorders mediated by these proteins can be studied using these compounds to perhaps initiate drug discovery efforts to develop novel therapeutic agents.

Mechanisms of fear learning and extinction: synaptic plasticity-fear memory connection

RATIONALE

The ability to memorize threat-associated cues and subsequently react to them, exhibiting escape or avoidance responses, is an essential, often life-saving behavioral mechanism that can be experimentally studied using the fear (threat) conditioning training paradigm. Presently, there is substantial evidence supporting the Synaptic Plasticity-Memory (SPM) hypothesis in relation to the mechanisms underlying the acquisition, retention, and extinction of conditioned fear memory.

OBJECTIVES

The purpose of this review article is to summarize findings supporting the SPM hypothesis in the context of conditioned fear control, applying the set of criteria and tests which were proposed as necessary to causally link lasting changes in synaptic transmission in corresponding neural circuits to fear memory acquisition and extinction with an emphasis on their pharmacological diversity.

RESULTS

The mechanisms of synaptic plasticity in fear circuits exhibit complex pharmacological profiles and satisfy all four SPM criteria-detectability, anterograde alteration, retrograde alteration, and mimicry.

CONCLUSION

The reviewed findings, accumulated over the last two decades, provide support for both necessity and sufficiency of synaptic plasticity in fear circuits for fear memory acquisition and retention, and, in part, for fear extinction, with the latter requiring additional experimental work.

Emerging Roles of Diacylglycerol-Sensitive TRPC4/5 Channels

Transient receptor potential classical or canonical 4 (TRPC4) and TRPC5 channels are members of the classical or canonical transient receptor potential (TRPC) channel family of non-selective cation channels. TRPC4 and TRPC5 channels are widely accepted as receptor-operated cation channels that are activated in a phospholipase C-dependent manner, following the G_q/11 protein-coupled receptor activation. However, their precise activation mechanism has remained largely elusive for a long time, as the TRPC4 and TRPC5 channels were considered as being insensitive to the second messenger diacylglycerol (DAG) in contrast to the other TRPC channels. Recent findings indicate that the C-terminal interactions with the scaffolding proteins Na⁺/H⁺ exchanger regulatory factor 1 and 2 (NHERF1 and NHERF2) dynamically regulate the DAG sensitivity of the TRPC4 and TRPC5 channels. Interestingly, the C-terminal NHERF binding suppresses, while the dissociation of NHERF enables, the DAG sensitivity of the TRPC4 and TRPC5 channels. This leads to the assumption that all of the TRPC channels are DAG sensitive. The identification of the regulatory function of the NHERF proteins in the TRPC4/5-NHERF protein complex offers a new starting point to get deeper insights into the molecular basis of TRPC channel activation. Future studies will have to unravel the physiological and pathophysiological functions of this multi-protein channel complex.

The research domain criteria framework in drug discovery for neuropsychiatric diseases: focus on negative valence

Drug discovery, particularly in the field of central nervous system, has had very limited success in the last few decades. A likely contributor is the poor translation between preclinical and clinical phases. The Research Domain Criteria of the National Institutes of Mental Health is a framework which aims to identify new ways of classifying mental illnesses that are based on observable behaviour and neurobiological measures, and to provide a guiding and evolving framework to improve the translation from preclinical to clinical research. At the core of the Research Domain Criteria approach is the assumption that the dimensional constructs described can be assessed across different units of analysis, thus enabling a more precise quantitative understanding of their neurobiological underpinnings, increasing the likelihood of identifying new and effective therapeutic approaches. In the present review, we discuss how the Research Domain Criteria can be applied to drug discovery with the domain Negative Valence, construct Potential Threat (‘Anxiety’) as an example. We will discuss the evidence supporting the utility of the Research Domain Criteria approach and evaluate how close we are to achieving a common thread of translational research from gene to self-report.

Diminishing fear: Optogenetic approach toward understanding neural circuits of fear control

Understanding complex behavioral processes, both learned and innate, requires detailed characterization of the principles governing signal flow in corresponding neural circuits. Previous studies were hampered by the lack of appropriate tools needed to address the complexities of behavior-driving micro- and macrocircuits. The development and implementation of optogenetic methodologies revolutionized the field of behavioral neuroscience, allowing precise spatiotemporal control of specific, genetically defined neuronal populations and their functional connectivity both in vivo and ex vivo, thus providing unprecedented insights into the cellular and network-level mechanisms contributing to behavior. Here, we review recent pioneering advances in behavioral studies with optogenetic tools, focusing on mechanisms of fear-related behavioral processes with an emphasis on approaches which could be used to suppress fear when it is pathologically expressed. We also discuss limitations of these methodologies as well as review new technological developments which could be used in future mechanistic studies of fear behavior.

Activation of TRPC1 Channel by Metabotropic Glutamate Receptor mGluR5 Modulates Synaptic Plasticity and Spatial Working Memory

Group I metabotropic glutamate receptors, in particular mGluR5, have been implicated in various forms of synaptic plasticity that are believed to underlie declarative memory. We observed that mGluR5 specifically activated a channel containing TRPC1, an isoform of the canonical family of transient receptor potential (TRPC) channels highly expressed in CA1-3 regions of the hippocampus. TRPC1 is able to form tetrameric complexes with TRPC4 and/or TRPC5 isoforms. TRPC1/4/5 complexes have recently been involved in the efficiency of synaptic transmission in the hippocampus. We therefore used a mouse model devoid of TRPC1 expression to investigate the involvement of mGluR5-TRPC1 pathway in synaptic plasticity and memory formation. Trpc1^-/- mice showed alterations in spatial working memory and fear conditioning. Activation of mGluR increased synaptic excitability in neurons from WT but not from Trpc1^-/- mice. LTP triggered by a theta burst could not maintain over time in brain slices from Trpc1^-/- mice. mGluR-induced LTD was also impaired in these mice. Finally, acute inhibition of TRPC1 by Pico145 on isolated neurons or on brain slices mimicked the genetic depletion of Trpc1 and inhibited mGluR-induced entry of cations and subsequent effects on synaptic plasticity, excluding developmental or compensatory mechanisms in Trpc1^-/- mice. In summary, our results indicate that TRPC1 plays a role in synaptic plasticity and spatial working memory processes.