Essential role for TRPC5 in amygdala function and fear-related behavior

Treatment with HC-070, a potent inhibitor of TRPC4 and TRPC5, leads to anxiolytic and antidepressant effects in mice

Background

Forty million adults in the US suffer from anxiety disorders, making these the most common forms of mental illness. Transient receptor potential channel canonical subfamily (TRPC) members 4 and 5 are non-selective cation channels highly expressed in regions of the cortex and amygdala, areas thought to be important in regulating anxiety. Previous work with null mice suggests that inhibition of TRPC4 and TRPC5 may have anxiolytic effects.

HC-070 in vitro

To assess the potential of TRPC4/5 inhibitors as an avenue for treatment, we invented a highly potent, small molecule antagonist of TRPC4 and TRPC5 which we call HC-070. HC-070 inhibits recombinant TRPC4 and TRPC5 homomultimers in heterologous expression systems with nanomolar potency. It also inhibits TRPC1/5 and TRPC1/4 heteromultimers with similar potency and reduces responses evoked by cholecystokinin tetrapeptide (CCK-4) in the amygdala. The compound is >400-fold selective over a wide range of molecular targets including ion channels, receptors, and kinases.

HC-070 in vivo

Upon oral dosing in mice, HC-070 achieves exposure levels in the brain and plasma deemed sufficient to test behavioral activity. Treatment with HC-070 attenuates the anxiogenic effect of CCK-4 in the elevated plus maze (EPM). The compound recapitulates the phenotype observed in both null TRPC4 and TRPC5 mice in a standard EPM. Anxiolytic and anti-depressant effects of HC-070 are also observed in pharmacological in vivo tests including marble burying, tail suspension and forced swim. Furthermore, HC-070 ameliorates the increased fear memory induced by chronic social stress. A careful evaluation of the pharmacokinetic-pharmacodynamic relationship reveals that substantial efficacy is observed at unbound brain levels similar to, or even lower than, the 50% inhibitory concentration (IC₅₀) recorded in vitro, increasing confidence that the observed effects are indeed mediated by TRPC4 and/or TRPC5 inhibition. Together, this experimental data set introduces a novel, high quality, small molecule antagonist of TRPC4 and TRPC5 containing channels and supports the targeting of TRPC4 and TRPC5 channels as a new mechanism of action for the treatment of psychiatric symptoms.

TrpC5 Mediates Acute Leptin and Serotonin Effects via Pomc Neurons

The molecular mechanisms underlying acute leptin and serotonin 2C receptor-induced hypophagia remain unclear. Here, we show that neuronal and pro-opiomelanocortin (Pomc)-specific loss of transient receptor potential cation 5 (TrpC5) subunits is sufficient to decrease energy expenditure and increase food intake resulting in elevated body weight. Deficiency of Trpc5 subunits in Pomc neurons is also sufficient to block the anorexigenic effects of leptin and serotonin 2C receptor (Ht2Cr) agonists. The loss of acute anorexigenic effects of these receptors is concomitant with a blunted electrophysiological response to both leptin and Ht2Cr agonists in arcuate Pomc neurons. We also demonstrate that the Ht2Cr agonist lorcaserin-induced improvements in glucose and insulin tolerance are blocked by TrpC5 deficiency in Pomc neurons. Together, our results link TrpC5 subunits in the brain with leptin- and serotonin 2C receptor-dependent changes in neuronal activity, as well as energy balance, feeding behavior, and glucose metabolism.

A small-molecule inhibitor of TRPC5 ion channels suppresses progressive kidney disease in animal models

Progressive kidney diseases are often associated with scarring of the kidney's filtration unit, a condition called focal segmental glomerulosclerosis (FSGS). This scarring is due to loss of podocytes, cells critical for glomerular filtration, and leads to proteinuria and kidney failure. Inherited forms of FSGS are caused by Rac1-activating mutations, and Rac1 induces TRPC5 ion channel activity and cytoskeletal remodeling in podocytes. Whether TRPC5 activity mediates FSGS onset and progression is unknown. We identified a small molecule, AC1903, that specifically blocks TRPC5 channel activity in glomeruli of proteinuric rats. Chronic administration of AC1903 suppressed severe proteinuria and prevented podocyte loss in a transgenic rat model of FSGS. AC1903 also provided therapeutic benefit in a rat model of hypertensive proteinuric kidney disease. These data indicate that TRPC5 activity drives disease and that TRPC5 inhibitors may be valuable for the treatment of progressive kidney diseases.

Na+ entry through heteromeric TRPC4/C1 channels mediates (-)Englerin A-induced cytotoxicity in synovial sarcoma cells

The sesquiterpene (-)Englerin A (EA) is an organic compound from the plant Phyllanthus engleri which acts via heteromeric TRPC4/C1 channels to cause cytotoxicity in some types of cancer cell but not normal cells. Here we identified selective cytotoxicity of EA in human synovial sarcoma cells (SW982 cells) and investigated the mechanism. EA induced cation channel current (Icat) in SW982 cells with biophysical characteristics of heteromeric TRPC4/C1 channels. Inhibitors of homomeric TRPC4 channels were weak inhibitors of the Icat and EA-induced cytotoxicity whereas a potent inhibitor of TRPC4/C1 channels (Pico145) strongly inhibited Icat and cytotoxicity. Depletion of TRPC1 converted Icat into a current with biophysical and pharmacological properties of homomeric TRPC4 channels and depletion of TRPC1 or TRPC4 suppressed the cytotoxicity of EA. A Na⁺/K⁺-ATPase inhibitor (ouabain) potentiated EA-induced cytotoxicity and direct Na⁺ loading by gramicidin-A caused Pico145-resistant cytotoxicity in the absence of EA. We conclude that EA has a potent cytotoxic effect on human synovial sarcoma cells which is mediated by heteromeric TRPC4/C1 channels and Na⁺ loading.

TRPC5 is required for the NO-dependent increase in dendritic Ca2+ and GABA release from chick retinal amacrine cells

GABAergic signaling from amacrine cells (ACs) is a fundamental aspect of visual signal processing in the inner retina. We have previously shown that nitric oxide (NO) can elicit release of GABA independently from activation of voltage-gated Ca²⁺ channels in cultured retinal ACs. This voltage-independent quantal GABA release relies on a Ca²⁺ influx mechanism with pharmacological characteristics consistent with the involvement of the transient receptor potential canonical (TRPC) channels TRPC4 and/or TRPC5. To determine the identity of these channels, we evaluated the ability of NO to elevate dendritic Ca²⁺ and to stimulate GABA release from cultured ACs under conditions known to alter the function of TRPC4 and 5. We found that these effects of NO are phospholipase C dependent, have a biphasic dependence on La³⁺, and are unaffected by moderate concentrations of the TRPC4-selective antagonist ML204. Together, these results suggest that NO promotes GABA release by activating TRPC5 channels in AC dendrites. To confirm a role for TRPC5, we knocked down the expression of TRPC5 using CRISPR/Cas9-mediated gene knockdown and found that both the NO-dependent Ca²⁺ elevations and increase in GABA release are dependent on the expression of TRPC5. These results demonstrate a novel NO-dependent mechanism for regulating neurotransmitter output from retinal ACs. NEW & NOTEWORTHY Elucidating the mechanisms regulating GABAergic synaptic transmission in the inner retina is key to understanding the flexibility of retinal ganglion cell output. Here, we demonstrate that nitric oxide (NO) can activate a transient receptor potential canonical 5 (TRPC5)-mediated Ca²⁺ influx, which is sufficient to drive vesicular GABA release from retinal amacrine cells. This NO-dependent mechanism can bypass the need for depolarization and may have an important role in processing the visual signal by enhancing retinal amacrine cell GABAergic inhibitory output.

Heteromeric channels formed by TRPC1, TRPC4 and TRPC5 define hippocampal synaptic transmission and working memory

Canonical transient receptor potential (TRPC) channels influence various neuronal functions. Using quantitative high-resolution mass spectrometry, we demonstrate that TRPC1, TRPC4, and TRPC5 assemble into heteromultimers with each other, but not with other TRP family members in the mouse brain and hippocampus. In hippocampal neurons from Trpc1/Trpc4/Trpc5-triple-knockout (Trpc1/4/5^-/-) mice, lacking any TRPC1-, TRPC4-, or TRPC5-containing channels, action potential-triggered excitatory postsynaptic currents (EPSCs) were significantly reduced, whereas frequency, amplitude, and kinetics of quantal miniature EPSC signaling remained unchanged. Likewise, evoked postsynaptic responses in hippocampal slice recordings and transient potentiation after tetanic stimulation were decreased. In vivo, Trpc1/4/5^-/- mice displayed impaired cross-frequency coupling in hippocampal networks and deficits in spatial working memory, while spatial reference memory was unaltered. Trpc1/4/5^-/- animals also exhibited deficiencies in adapting to a new challenge in a relearning task. Our results indicate the contribution of heteromultimeric channels from TRPC1, TRPC4, and TRPC5 subunits to the regulation of mechanisms underlying spatial working memory and flexible relearning by facilitating proper synaptic transmission in hippocampal neurons.

The Role of TRPC6 in the Neuroprotection of Calycosin Against Cerebral Ischemic Injury

Our previous studies have provided evidences that calycosin can protect the brain from ischemia/reperfusion injury, but its mechanisms is not fully understand. Transient receptor potential canonical 6 (TRPC6) has a critical role in promoting neuronal survival against cerebral ischemic injury. The aim of the present study is to test whether calycosin protects against cerebral ischemic injury through TRPC6-CREB pathway. In vivo, rats were subjected to transient middle cerebral artery occlusion (MCAO) for 2 h and then treated with different doses of calycosin at the onset of reperfusion. In vitro, primary cultured neurons were treated by calycosin, then exposed to 2 h oxygen glucose deprivation (OGD) followed by 24 h reoxygenation. Our results showed that treatment with calycosin protected against ischemia-induced damages by increasing TRPC6 and P-CREB expression and inhibiting calpain activation. The neuroprotection effect of calycosin was diminished by inhibition or knockdown of TRPC6 and CREB. These findings indicated that the potential neuroprotection mechanism of calycosin was involved with TRPC6-CREB pathway.

A benzothiadiazine derivative and methylprednisolone are novel and selective activators of transient receptor potential canonical 5 (TRPC5) channels

The transient receptor potential canonical channel 5 (TRPC5) is a Ca²⁺-permeable ion channel, which is predominantly expressed in the brain. TRPC5-deficient mice exhibit a reduced innate fear response and impaired motor control. In addition, outgrowth of hippocampal and cerebellar neurons is retarded by TRPC5. However, pharmacological evidence of TRPC5 function on cellular or organismic levels is sparse. Thus, there is still a need for identifying novel and efficient TRPC5 channel modulators. We, therefore, screened compound libraries and identified the glucocorticoid methylprednisolone and N-[3-(adamantan-2-yloxy)propyl]-3-(6-methyl-1,1-dioxo-2H-1λ⁶,2,4-benzothiadiazin-3-yl)propanamide (BTD) as novel TRPC5 activators. Comparisons with closely related chemical structures from the same libraries indicate important substructures for compound efficacy. Methylprednisolone activates TRPC5 heterologously expressed in HEK293 cells with an EC₅₀ of 12μM, while BTD-induced half-maximal activation is achieved with 5-fold lower concentrations, both in Ca²⁺ assays (EC₅₀=1.4μM) and in electrophysiological whole cell patch clamp recordings (EC₅₀=1.3 μM). The activation resulting from both compounds is long lasting, reversible and sensitive to clemizole, a recently established TRPC5 inhibitor. No influence of BTD on homotetrameric members of the remaining TRPC family was observed. On the main sensory TRP channels (TRPA1, TRPV1, TRPM3, TRPM8) BTD exerts only minor activity. Furthermore, BTD can activate heteromeric channel complexes consisting of TRPC5 and its closest relatives TRPC1 or TRPC4, suggesting a high selectivity of BTD for channel complexes bearing at least one TRPC5 subunit.

TRPC1 is a differential regulator of hypoxia-mediated events and Akt signalling in PTEN-deficient breast cancer cells

Hypoxia is a feature of the tumour microenvironment that promotes invasiveness, resistance to chemotherapeutics and cell survival. Our studies identify the transient receptor potential canonical-1 (TRPC1) ion channel as a key component of responses to hypoxia in breast cancer cells. This regulation includes control of specific epithelial to mesenchymal transition (EMT) events and hypoxia-mediated activation of signalling pathways such as activation of the EGFR, STAT3 and the autophagy marker LC3B, through hypoxia-inducible factor-1α (HIF1α)-dependent and -independent mechanisms. TRPC1 regulated HIF1α levels in PTEN-deficient MDA-MB-468 and HCC1569 breast cancer cell lines. This regulation arises from effects on the constitutive translation of HIF1α under normoxic conditions via an Akt-dependent pathway. In further support of the role of TRPC1 in EMT, its expression is closely associated with EMT- and metastasis-related genes in breast tumours, and is enhanced in basal B breast cancer cell lines. TRPC1 expression is also significantly prognostic for basal breast cancers, particularly those classified as lymph node positive. The defined roles of TRPC1 identified here could be therapeutically exploited for the control of oncogenic pathways in breast cancer cells.

Transient receptor potential canonical 5 channels plays an essential role in hepatic dyslipidemia associated with cholestasis

Transient receptor potential canonical 5 (TRPC5), a calcium-permeable, non-selective cation channel is expressed in the periphery, but there is limited knowledge of its regulatory roles in vivo. Endogenous modulators of TRPC5 include a range of phospholipids that have an established role in liver disease, including lysophosphatidylcholine (LPC). Cholestasis is characterized by impairment of excretion of bile acids, leading to elevation of hepatic bile acids. We investigated the contribution of TRPC5 in a murine model of cholestasis. Wild-type (WT) and TRPC5 knock-out (KO) mice were fed a diet supplemented with 0.5% cholic acid (CA) for 21 days. CA-diet supplementation resulted in enlargement of the liver in WT mice, which was ameliorated in TRPC5 KO mice. Hepatic bile acid and lipid content was elevated in WT mice, with a reduction observed in TRPC5 KO mice. Consistently, liver enzymes were significantly increased in cholestatic WT mice and significantly blunted in TRPC5 KO mice. Localized dyslipidaemia, secondary to cholestasis, was investigated utilizing a selected lipid analysis. This revealed significant perturbations in the lipid profile following CA-diet feeding, with increased cholesterol, triglycerides and phospholipids, in WT, but not TRPC5 KO mice. Our results suggest that activation of TRPC5 contributes to the development of cholestasis and associated dyslipidemia. Modulation of TRPC5 activity may present as a novel therapeutic target for liver disease.

Functional Characterization of Transient Receptor Potential (TRP) Channel C5 in Female Murine Gonadotropes

Gonadotrope cells in the anterior pituitary gland secrete gonadotropins regulating gonadal function in mammals. Recent results have implicated transient receptor potential (TRP) cation channels in pituitary physiology; however, if and how TRP channels contribute to gonadotrope function is not known. Here, we report that 14 out of 28 TRP channels encoded in the mouse genome are expressed in murine gonadotropes with highest expression levels found for canonical TRP (TRPC) channel 5 in juvenile females. We show that TRP channel expression in these cells exhibits considerable plasticity and that it depends on the sex and the developmental and hormonal status of the animal. We then combine different genetic strategies including genetic confocal Ca2+ imaging in whole-mount pituitary gland preparations to characterize TRPC5 channel function in gonadotropes from juvenile females. We show that the TRPC5 agonist Englerin A activates a cytosolic Ca2+ signal and a whole-cell current in these cells, which is absent in TRPC5-deficient mice, and demonstrate that TRPC5 forms functional heteromultimers with TRPC1 in gonadotropes. We further show that the Englerin A-activated TRPC5-dependent Ca2+ signal is mediated by Ca2+ influx both via TRPC5 and via l-type voltage-gated Ca2+ channels, activated by the depolarization through TRPC5-mediated cation influx. Finally, we demonstrate that the gonadotropin-releasing hormone (GnRH)-mediated net depolarization is significantly reduced in gonadotropes isolated from TRPC5-deficient mice. In conclusion, our data suggest that TRPC5 contributes to depolarization of the plasma membrane in gonadotropes upon GnRH stimulation and increases the intracellular Ca2+ concentration via its own Ca2+ permeability and via the activation of voltage-gated Ca2+ channels.

Picomolar, selective, and subtype-specific small-molecule inhibition of TRPC1/4/5 channels

The concentration of free cytosolic Ca²⁺ and the voltage across the plasma membrane are major determinants of cell function. Ca²⁺-permeable non-selective cationic channels are known to regulate these parameters, but understanding of these channels remains inadequate. Here we focus on transient receptor potential canonical 4 and 5 proteins (TRPC4 and TRPC5), which assemble as homomers or heteromerize with TRPC1 to form Ca²⁺-permeable non-selective cationic channels in many mammalian cell types. Multiple roles have been suggested, including in epilepsy, innate fear, pain, and cardiac remodeling, but limitations in tools to probe these channels have restricted progress. A key question is whether we can overcome these limitations and develop tools that are high-quality, reliable, easy to use, and readily accessible for all investigators. Here, through chemical synthesis and studies of native and overexpressed channels by Ca²⁺ and patch-clamp assays, we describe compound 31, a remarkable small-molecule inhibitor of TRPC1/4/5 channels. Its potency ranged from 9 to 1300 pm, depending on the TRPC1/4/5 subtype and activation mechanism. Other channel types investigated were unaffected, including TRPC3, TRPC6, TRPV1, TRPV4, TRPA1, TRPM2, TRPM8, and store-operated Ca²⁺ entry mediated by Orai1. These findings suggest identification of an important experimental tool compound, which has much higher potency for inhibiting TRPC1/4/5 channels than previously reported agents, impressive specificity, and graded subtype selectivity within the TRPC1/4/5 channel family. The compound should greatly facilitate future studies of these ion channels. We suggest naming this TRPC1/4/5-inhibitory compound Pico145.

TRPC Channels and Mental Disorders

Transient receptor potential canonical (TRPC) channels mediate the influx of different types of cations through the cell membrane and are involved in many functions of the organism. Evidences of involvement of TRPC channels in neuronal development suggest that this family of proteins might play a role in certain neurological disorders. As reported, knockout mice for different TRPC channels show alterations in neuronal morphological and functional parameters, with behavioral abnormalities, such as in exploratory and social behaviors. Although mutations in TRPC channels could be related to mental/neurological disorders, there are only a few cases reported in literature, indicating that this correlation should be further explored. Nonetheless, other functional evidences support the implication of these channels in neurological diseases. In this chapter, we summarize the main findings relating TRPC channels to neurological disorders, such as autism spectrum disorders, bipolar disorder, and intellectual disability among others.

TRPC Channels and Programmed Cell Death

Neurotrophins, including nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), bind to their high-affinity receptors to promote neuronal survival during brain development. One of the key downstream pathways is the phospholipase C (PLC) pathway, which not only plays a central role in calcium release from internal store but also in activation of TRPC channels coupled with neurotrophin receptors. TRPC channels are required for the neurotrophin-mediated neuronal protective effects. In addition, activation of TRPC channels is able to protect neurons in the absence of neurotrophin. In some circumstances, TRPC channels coupled with metabotropic glutamate receptor may mediate the excitotoxicity by calcium overload. One of the key questions in the field is the channel gating mechanisms; understanding of which would help design compounds to modulate the channel properties. The development and identification of TRPC channel agonists or blockers are promising and may unveil new therapeutic drugs for the treatment of neurodegenerative diseases and epilepsy.

Dynamic NHERF interaction with TRPC4/5 proteins is required for channel gating by diacylglycerol

The activation mechanism of the classical transient receptor potential channels TRPC4 and -5 via the G_q/11 protein-phospholipase C (PLC) signaling pathway has remained elusive so far. In contrast to all other TRPC channels, the PLC product diacylglycerol (DAG) is not sufficient for channel activation, whereas TRPC4/5 channel activity is potentiated by phosphatidylinositol 4,5-bisphosphate (PIP₂) depletion. As a characteristic structural feature, TRPC4/5 channels contain a C-terminal PDZ-binding motif allowing for binding of the scaffolding proteins Na⁺/H⁺ exchanger regulatory factor (NHERF) 1 and 2. PKC inhibition or the exchange of threonine for alanine in the C-terminal PDZ-binding motif conferred DAG sensitivity to the channel. Altogether, we present a DAG-mediated activation mechanism for TRPC4/5 channels tightly regulated by NHERF1/2 interaction. PIP₂ depletion evokes a C-terminal conformational change of TRPC5 proteins leading to dynamic dissociation of NHERF1/2 from the C terminus of TRPC5 as a prerequisite for DAG sensitivity. We show that NHERF proteins are direct regulators of ion channel activity and that DAG sensitivity is a distinctive hallmark of TRPC channels.

Molecular Determinants of the Sensitivity to Gq/11-Phospholipase C-dependent Gating, Gd3+ Potentiation, and Ca2+ Permeability in the Transient Receptor Potential Canonical Type 5 (TRPC5) Channel

Transient receptor potential canonical type 5 (TRPC5) is a Ca²⁺-permeable cation channel that is highly expressed in the brain and is implicated in motor coordination, innate fear behavior, and seizure genesis. The channel is activated by a signal downstream of the G-protein-coupled receptor (GPCR)-G_q/11-phospholipase C (PLC) pathway. In this study we aimed to identify the molecular mechanisms involved in regulating TRPC5 activity. We report that Arg-593, a residue located in the E4 loop near the TRPC5 extracellular Gd³⁺ binding site, is critical for conferring the sensitivity to GPCR-G_q/11-PLC-dependent gating on TRPC5. Indeed, guanosine 5'-O-(thiotriphosphate) and GPCR agonists only weakly activate the TRPC5_R593A mutant, whereas the addition of Gd³⁺ rescues the mutant's sensitivity to GPCR-G_q/11-PLC-dependent gating. Computer modeling suggests that Arg-593 may cross-bridge the E3 and E4 loops, forming the "molecular fulcrum." While validating the model using site-directed mutagenesis, we found that the Tyr-542 residue is critical for establishing a functional Gd³⁺ binding site, the Tyr-541 residue participates in fine-tuning Gd³⁺-sensitivity, and that the Asn-584 residue determines Ca²⁺ permeability of the TRPC5 channel. This is the first report providing molecular insights into the molecular mechanisms regulating the sensitivity to GPCR-G_q/11-PLC-dependent gating of a receptor-operated channel.

TRPC5 channels participate in pressure-sensing in aortic baroreceptors

Blood pressure is maintained within a normal physiological range by a sophisticated regulatory mechanism. Baroreceptors serve as a frontline sensor to detect the change in blood pressure. Nerve signals are then sent to the cardiovascular control centre in the brain in order to stimulate baroreflex responses. Here, we identify TRPC5 channels as a mechanical sensor in aortic baroreceptors. In Trpc5 knockout mice, the pressure-induced action potential firings in the afferent nerve and the baroreflex-mediated heart rate reduction are attenuated. Telemetric measurements of blood pressure demonstrate that Trpc5 knockout mice display severe daily blood pressure fluctuation. Our results suggest that TRPC5 channels represent a key pressure transducer in the baroreceptors and play an important role in maintaining blood pressure stability. Because baroreceptor dysfunction contributes to a variety of cardiovascular diseases including hypertension, heart failure and myocardial infarction, our findings may have important future clinical implications.

The identity of mechanosensors within aortic baroreceptors that sense fluctuations in blood pressure is unclear. Here, Lau et al. show that a cation channel TRPC5 acts as a transducer of mechanical stimuli in aortic baroreceptor neurons in rodents.

Psychiatric Disorders and TRP Channels: Focus on Psychotropic Drugs

Psychiatric and neurological disorders are mostly associated with the changes in neural calcium ion signaling pathways required for activity-triggered cellular events. One calcium channel family is the TRP cation channel family, which contains seven subfamilies. Results of recent papers have discovered that calcium ion influx through TRP channels is important. We discuss the latest advances in calcium ion influx through TRP channels in the etiology of psychiatric disorders. Activation of TRPC4, TRPC5, and TRPV1 cation channels in the etiology of psychiatric disorders such as anxiety, fear-associated responses, and depression modulate calcium ion influx. Evidence substantiates that anandamide and its analog (methanandamide) induce an anxiolytic-like effect via CB1 receptors and TRPV1 channels. Intracellular calcium influx induced by oxidative stress has an significant role in the etiology of bipolar disorders (BDs), and studies recently reported the important role of TRP channels such as TRPC3, TRPM2, and TRPV1 in converting oxidant or nitrogen radical signaling to cytosolic calcium ion homeostasis in BDs. The TRPV1 channel also plays a function in morphine tolerance and hyperalgesia. Among psychotropic drugs, amitriptyline and capsazepine seem to have protective effects on psychiatric disorders via the TRP channels. Some drugs such as cocaine and methamphetamine also seem to have an important role in alcohol addiction and substance abuse via activation of the TRPV1 channel. Thus, we explore the relationships between the etiology of psychiatric disorders and TRP channel-regulated mechanisms. Investigation of the TRP channels in psychiatric disorders holds the promise of the development of new drug treatments.

Transient receptor potential canonical 5 (TRPC5) protects against pain and vascular inflammation in arthritis and joint inflammation

Objective

Transient receptor potential canonical 5 (TRPC5) is functionally expressed on a range of cells including fibroblast-like synoviocytes, which play an important role in arthritis. A role for TRPC5 in inflammation has not been previously shown in vivo. We investigated the contribution of TRPC5 in arthritis.

Methods

Male wild-type and TRPC5 knockout (KO) mice were used in a complete Freund's adjuvant (CFA)-induced unilateral arthritis model, assessed over 14 days. Arthritis was determined by measurement of knee joint diameter, hindlimb weightbearing asymmetry and pain behaviour. Separate studies involved chronic pharmacological antagonism of TRPC5 channels. Synovium from human postmortem control and inflammatory arthritis samples were investigated for TRPC5 gene expression.

Results

At baseline, no differences were observed. CFA-induced arthritis resulted in increased synovitis in TRPC5 KO mice assessed by histology. Additionally, TRPC5 KO mice demonstrated reduced ispilateral weightbearing and nociceptive thresholds (thermal and mechanical) following CFA-induced arthritis. This was associated with increased mRNA expression of inflammatory mediators in the ipsilateral synovium and increased concentration of cytokines in synovial lavage fluid. Chronic treatment with ML204, a TRPC5 antagonist, augmented weightbearing asymmetry, secondary hyperalgesia and cytokine concentrations in the synovial lavage fluid. Synovia from human inflammatory arthritis demonstrated a reduction in TRPC5 mRNA expression.

Conclusions

Genetic deletion or pharmacological blockade of TRPC5 results in an enhancement in joint inflammation and hyperalgesia. Our results suggest that activation of TRPC5 may be associated with an endogenous anti-inflammatory/analgesic pathway in inflammatory joint conditions.

Calcium-permeable ion channels in the kidney

Calcium ions (Ca(2+)) are crucial for a variety of cellular functions. The extracellular and intracellular Ca(2+) concentrations are thus tightly regulated to maintain Ca(2+) homeostasis. The kidney, one of the major organs of the excretory system, regulates Ca(2+) homeostasis by filtration and reabsorption. Approximately 60% of the Ca(2+) in plasma is filtered, and 99% of that is reabsorbed by the kidney tubules. Ca(2+) is also a critical signaling molecule in kidney development, in all kidney cellular functions, and in the emergence of kidney diseases. Recently, studies using genetic and molecular biological approaches have identified several Ca(2+)-permeable ion channel families as important regulators of Ca(2+) homeostasis in kidney. These ion channel families include transient receptor potential channels (TRP), voltage-gated calcium channels, and others. In this review, we provide a brief and systematic summary of the expression, function, and pathological contribution for each of these Ca(2+)-permeable ion channels. Moreover, we discuss their potential as future therapeutic targets.

Ephrin-B3 coordinates timed axon targeting and amygdala spinogenesis for innate fear behaviour

Innate emotion response to environmental stimuli is a fundamental brain function that is controlled by specific neural circuits. Dysfunction of early emotional circuits may lead to neurodevelopmental disorders such as autism and schizophrenia. However, how the functional circuits are formed to prime initial emotional behaviours remain elusive. We reveal here using gene-targeted mutations an essential role for ephrin-B3 ligand-like activity in the development of innate fear in the neonatal brain. We further demonstrate that ephrin-B3 controls axon targeting and coordinates spinogenesis and neuronal activity within the amygdala. The morphological and behavioural abnormalities in ephrin-B3 mutant mice are rescued by conditional knock-in of wild-type ephrin-B3 during the critical period when axon targeting and fear responses are initiated. Our results thus define a key axonal molecule that participates in the wiring of amygdala circuits and helps bring about fear emotion during the important adolescence period.

Natural and synthetic flavonoid modulation of TRPC5 channels

Background and Purpose

The TRPC5 proteins assemble to create calcium‐permeable, non‐selective, cationic channels. We sought novel modulators of these channels through studies of natural products.

Experimental Approach

Intracellular calcium measurements and patch clamp recordings were made from cell lines. Compounds were generated by synthetic chemistry.

Key Results

Through a screen of natural products used in traditional Chinese medicines, the flavonol galangin was identified as an inhibitor of lanthanide‐evoked calcium entry in TRPC5 overexpressing HEK 293 cells (IC₅₀ 0.45 μM). Galangin also inhibited lanthanide‐evoked TRPC5‐mediated current in whole‐cell and outside‐out patch recordings. In differentiated 3T3‐L1 cells, it inhibited constitutive and lanthanide‐evoked calcium entry through endogenous TRPC5‐containing channels. The related natural flavonols, kaempferol and quercetin were less potent inhibitors of TRPC5. Myricetin and luteolin lacked effect, and apigenin was a stimulator. Based on structure–activity relationship studies with natural and synthetic flavonols, we designed 3,5,7‐trihydroxy‐2‐(2‐bromophenyl)‐4H‐chromen‐4‐one (AM12), which inhibited lanthanide‐evoked TRPC5 activity with an IC₅₀ of 0.28 μM. AM12 also inhibited TRPC5 activity evoked by the agonist (−)‐Englerin A and was effective in excised outside‐out membrane patches, suggesting a relatively direct effect. It inhibited TRPC4 channels similarly, but its inhibitory effect on TRPC1–TRPC5 heteromeric channels was weaker.

Conclusions and Implications

The data suggest that galangin (a natural product from the ginger family) is a TRPC5 inhibitor and that other natural and synthetic flavonoids contain antagonist or agonist capabilities at TRPC5 and closely related channels depending on the substitution patterns of both the chromone core and the phenyl ring.

Transient receptor potential channel C5 in cancer chemoresistance

The transient receptor potential (TRP) superfamily contains at least 28 homologs in mammalian. These proteins form TRP channels are permeable to monovalent and divalent cations and participate in a variety of physiological functions. Dysregulation of TRP channels is responsible for numerous diseases. This review provides a brief short overview of mammalian TRP channels with a focus on TRPC5 and its role in cancers. Dysregulation of TRPC5 interrupts Ca(2+) homeostasis in cancer cells, which activates signaling pathways that are highly associated with cancer progression, especially cancer chemoresistance. Based on the important role of TRPC5, we also discuss the potential of TRPC5 as a target for therapeutic intervention. Either direct targeting of TRPC5 or indirect interruption of TRPC5-related signaling pathways may effectively overcome cancer chemoresistance.

Oxytocin Stimulates Extracellular Ca2+ Influx Through TRPV2 Channels in Hypothalamic Neurons to Exert Its Anxiolytic Effects

There is growing interest in anxiolytic and pro-social effects of the neuropeptide oxytocin (OXT), but the underlying intraneuronal mechanisms are largely unknown. Here we examined OXT-mediated anxiolysis in the hypothalamic paraventricular nucleus (PVN) of rats and effects of OXT administration on signaling events in hypothalamic primary and immortalized cells. In vivo, the application of SKF96365 prevented the anxiolytic activity of OXT in the PVN, suggesting that changes in intracellular Ca(2+) mediate the acute OXT behavioral effects. In vitro, mainly in the neurons with autonomous Ca(2+) oscillations, OXT increased intracellular Ca(2+) concentration and oscillation amplitude. Pharmacological intervention revealed OXT-dependent changes in Ca(2+) signaling that required activation of transient receptor potential vanilloid type-2 channel (TRPV2), mediated by phosphoinositide 3-kinase. TRPV2 induced the activation of the anxiolytic mitogen-activated protein kinase kinase (MEK1/2). In situ, immunohistochemistry revealed co-localization of TRPV2 and OXT in the PVN. Thus, functional and pharmacological analyses identified TRPV2 as a mediator of anxiolytic effects of OXT, conveying the OXT signal to MEK1/2 via modulation of intracellular Ca(2+).

Assessment of ALK status by FISH on 1000 Spanish non-small cell lung cancer patients

INTRODUCTION

Patients with non-small cell lung cancer (NSCLC) harboring anaplastic lymphoma kinase (ALK) rearrangement selectively respond to ALK inhibitors. Thus, identification of ALK rearrangements has become a standard diagnostic test in advanced NSCLC patients. Our institution has been a referral center in Spain for ALK determination by Fluorescent in situ hybridization (FISH). The aim of our study was to assess the feasibility and the FISH patterns of the ALK gene and to evaluate the clinical and pathological features of patients with ALK alterations.

METHODS

Between 2010 and 2014, 1092 samples were evaluated for ALK using FISH technique (927 histological samples, 165 cytological samples). Correlation with available clinical-pathological information was assessed.

RESULTS

ALK rearrangement was found in 35 patients (3.2%). Cytological samples (using either direct smears or cell blocks), were more frequently non-assessable than histological samples (69% versus 89%, respectively) (p < 0.001). Within the ALK-rearranged cases the majority were female, non-smokers, and stage IV.

CONCLUSIONS

Although assessable in cytological samples, biopsies are preferred when available for ALK evaluation by FISH. The ALK translocation prevalence and the associated clinico-pathological features in Spanish NSCLC patients are similar to those previously reported.

Serotonin stimulates lateral habenula via activation of the post-synaptic serotonin 2/3 receptors and transient receptor potential channels

There is growing interest on the role of the lateral habenula (LHb) in depression, because it closely and bilaterally connects with the serotoninergic raphe nuclei. The LHb sends glutamate efferents to the raphe nuclei, while it receives serotoninergic afferents, and expresses a high density of serotonin (5-HT) receptors. Recent studies suggest that 5-HT receptors exist both in the presynaptic and postsynaptic sites of LHb neurons, and activation of these receptors may have different effects on the activity of LHb neurons. The current study focused on the effect of 5-HT on the postsynaptic membrane. We found that 5-HT initiated a depolarizing inward current (I((5-HTi))) and accelerated spontaneous firing in ∼80% of LHb neurons in rat brain slices. I((5-HTi)) was also induced by the 5-HT uptake blocker citalopram, indicating activity of endogenous 5-HT. I((5-HTi)) was diminished by 5-HT(2/3) receptor antagonists (ritanserin, SB-200646 or ondansetron), and activated by the selective 5-HT(2/3) agonists 1-(3-Chlorophenyl) piperazine hydrochloride or 1-(3-Chlorophenyl) biguanide hydrochloride. Furthermore, I((5-HTi)) was attenuated by 2-Aminoethyl diphenylborinate, a blocker of transient receptor potential channels, and an IP3 receptor inhibitor, indicating the involvement of transient receptor potential channels. These results demonstrate that the reciprocal connection between the LHb and the 5-HT system highlights a key role for 5-HT stimulation of LHb neurons that may be important in the pathogenesis of depression.

Increased TRPC5 glutathionylation contributes to striatal neuron loss in Huntington's disease

Aberrant glutathione or Ca(2+) homeostasis due to oxidative stress is associated with the pathogenesis of neurodegenerative disorders. The Ca(2+)-permeable transient receptor potential cation (TRPC) channel is predominantly expressed in the brain, which is sensitive to oxidative stress. However, the role of the TRPC channel in neurodegeneration is not known. Here, we report a mechanism of TRPC5 activation by oxidants and the effect of glutathionylated TRPC5 on striatal neurons in Huntington's disease. Intracellular oxidized glutathione leads to TRPC5 activation via TRPC5 S-glutathionylation at Cys176/Cys178 residues. The oxidized glutathione-activated TRPC5-like current results in a sustained increase in cytosolic Ca(2+), activated calmodulin-dependent protein kinase and the calpain-caspase pathway, ultimately inducing striatal neuronal cell death. We observed an abnormal glutathione pool indicative of an oxidized state in the striatum of Huntington's disease transgenic (YAC128) mice. Increased levels of endogenous TRPC5 S-glutathionylation were observed in the striatum in both transgenic mice and patients with Huntington's disease. Both knockdown and inhibition of TRPC5 significantly attenuated oxidation-induced striatal neuronal cell death. Moreover, a TRPC5 blocker improved rearing behaviour in Huntington's disease transgenic mice and motor behavioural symptoms in littermate control mice by increasing striatal neuron survival. Notably, low levels of TRPC1 increased the formation of TRPC5 homotetramer, a highly Ca(2+)-permeable channel, and stimulated Ca(2+)-dependent apoptosis in Huntington's disease cells (STHdh(Q111/111)). Taken together, these novel findings indicate that increased TRPC5 S-glutathionylation by oxidative stress and decreased TRPC1 expression contribute to neuronal damage in the striatum and may underlie neurodegeneration in Huntington's disease.

The role of TRPC6 in seizure susceptibility and seizure-related neuronal damage in the rat dentate gyrus

Transient receptor potential canonical channel-6 (TRPC6) forms Ca(2+)-permeable non-selective cation channels in neurons. Although TRPC6 plays an important role in neurite outgrowth and neuronal survival during development, TRPC6 expression profiles available to identify distinctive hippocampal neuronal damage and hippocampal excitability in epilepsy are less defined. As compared to normal animals, TRPC6 expression was down-regulated in chronic epileptic rats showing spontaneous recurrent seizures. TRPC6 knockdown increased seizure susceptibility, excitability ratio and paired-pulse inhibition in the dentate gyrus (DG) of normal animals. Furthermore, TRPC6 knockdown promoted programmed neuronal necrosis in dentate granule cells, but prevented it in CA1 and CA3 neurons following status epilepticus. The present data suggest for the first time that TRPC6 may inhibit seizure susceptibility and neuronal vulnerability in the rat DG.

Acute Treatment with a Novel TRPC4/C5 Channel Inhibitor Produces Antidepressant and Anxiolytic-Like Effects in Mice

Transient receptor potential canonical (TRPC) channels are widely expressed in brain and involved in various aspects of brain function. Both TRPC4 and TRPC5 have been implicated in innate fear function, which represents a key response to environmental stress. However, to what extent the TRPC4/C5 channels are involved in psychiatric disorders remains unexplored. Here, we tested the antidepressant and anxiolytic-like effects of a newly identified TRPC4/C5 inhibitor, M084. We show that a single intraperitoneal administration of M084 at 10 mg/kg body weight to C57BL/6 male mice significantly shortened the immobility time in forced swim test and tail suspension test within as short as 2 hours. The M084-treated mice spent more time exploring in illuminated and open areas in light/dark transition test and elevated plus maze test. In mice subjected to chronic unpredictable stress, M084 treatment reversed the enhanced immobility time in forced swim test and decreased the latency to feed in novelty suppressed feeding test. The treatment of M084 increased BDNF expression in both mRNA and protein levels, as well as phosphorylation levels of AKT and ERK, in prefrontal cortex. Our results indicate that M084 exerts rapid antidepressant and anxiolytic-like effects at least in part by acting on BDNF and its downstream signaling. We propose M084 as a lead compound for further druggability research.

Englerin A Agonizes the TRPC4/C5 Cation Channels to Inhibit Tumor Cell Line Proliferation

Englerin A is a structurally unique natural product reported to selectively inhibit growth of renal cell carcinoma cell lines. A large scale phenotypic cell profiling experiment (CLiP) of englerin A on ¬over 500 well characterized cancer cell lines showed that englerin A inhibits growth of a subset of tumor cell lines from many lineages, not just renal cell carcinomas. Expression of the TRPC4 cation channel was the cell line feature that best correlated with sensitivity to englerin A, suggesting the hypothesis that TRPC4 is the efficacy target for englerin A. Genetic experiments demonstrate that TRPC4 expression is both necessary and sufficient for englerin A induced growth inhibition. Englerin A induces calcium influx and membrane depolarization in cells expressing high levels of TRPC4 or its close ortholog TRPC5. Electrophysiology experiments confirmed that englerin A is a TRPC4 agonist. Both the englerin A induced current and the englerin A induced growth inhibition can be blocked by the TRPC4/C5 inhibitor ML204. These experiments confirm that activation of TRPC4/C5 channels inhibits tumor cell line proliferation and confirms the TRPC4 target hypothesis generated by the cell line profiling. In selectivity assays englerin A weakly inhibits TRPA1, TRPV3/V4, and TRPM8 which suggests that englerin A may bind a common feature of TRP ion channels. In vivo experiments show that englerin A is lethal in rodents near doses needed to activate the TRPC4 channel. This toxicity suggests that englerin A itself is probably unsuitable for further drug development. However, since englerin A can be synthesized in the laboratory, it may be a useful chemical starting point to identify novel modulators of other TRP family channels.

The role of polyunsaturated fatty acids and GPR40 receptor in brain

Polyunsaturated fatty acids (PUFAs) are found in abundance in the nervous system. They perform significant functions for example boosting synaptogenesis, neurogenesis, inducing antinociception, stimulating gene expression and neuronal activity, preventing apoptosis and neuroinflammation. G-protein-coupled receptor 40 (GPR40), also called free fatty acid receptor 1 (FFA1), is ubiquitously expressed in various regions of the human brain including the olfactory bulb, midbrain, medulla oblongata, hippocampus, hypothalamus, cerebral cortex, cerebellum and in the spinal cord. GPR40, when binding with polyunsaturated fatty acids (PUFAs) has shown promising therapeutic potential. This review presents current knowledge regarding the pharmacological properties of GPR40 and addresses its functions in brain, with a focus on neurodevelopment & neurogenesis. Furthermore, the demonstration of GPR40 involvement in several neuropathological conditions such as apoptosis, inflammatory pain, Alzheimer's disease and Parkinson's disease. Although the results are encouraging, further research is needed to clarify their role in the treatment of inflammatory pain, Alzheimer's disease and Parkinson's disease. This article is part of the Special Issue entitled 'Lipid Sensing G Protein-Coupled Receptors in the CNS'.

Identification and optimization of 2-aminobenzimidazole derivatives as novel inhibitors of TRPC4 and TRPC5 channels

BACKGROUND AND PURPOSE

Transient receptor potential canonical (TRPC) channels play important roles in a broad array of physiological functions and are involved in various diseases. However, due to a lack of potent subtype-specific inhibitors the exact roles of TRPC channels in physiological and pathophysiological conditions have not been elucidated.

EXPERIMENTAL APPROACH

Using fluorescence membrane potential and Ca(2+) assays and electrophysiological recordings, we characterized new 2-aminobenzimidazole-based small molecule inhibitors of TRPC4 and TRPC5 channels identified from cell-based fluorescence high-throughput screening.

KEY RESULTS

The original compound, M084, was a potent inhibitor of both TRPC4 and TRPC5, but was also a weak inhibitor of TRPC3. Structural modifications of the lead compound resulted in the identification of analogues with improved potency and selectivity for TRPC4 and TRPC5 channels. The aminobenzimidazole derivatives rapidly inhibited the TRPC4- and TRPC5-mediated currents when applied from the extracellular side and this inhibition was independent of the mode of activation of these channels. The compounds effectively blocked the plateau potential mediated by TRPC4-containing channels in mouse lateral septal neurons, but did not affect the activity of heterologously expressed TRPA1, TRPM8, TRPV1 or TRPV3 channels or that of the native voltage-gated Na(+) , K(+) and Ca(2) (+) channels in dissociated neurons.

CONCLUSIONS AND IMPLICATIONS

The TRPC4/C5-selective inhibitors developed here represent novel and useful pharmaceutical tools for investigation of physiological and pathophysiological functions of TRPC4/C5 channels.

Plasma membrane mechanical stress activates TRPC5 channels

Mechanical forces exerted on cells impose stress on the plasma membrane. Cells sense this stress and elicit a mechanoelectric transduction cascade that initiates compensatory mechanisms. Mechanosensitive ion channels in the plasma membrane are responsible for transducing the mechanical signals to electrical signals. However, the mechanisms underlying channel activation in response to mechanical stress remain incompletely understood. Transient Receptor Potential (TRP) channels serve essential functions in several sensory modalities. These channels can also participate in mechanotransduction by either being autonomously sensitive to mechanical perturbation or by coupling to other mechanosensory components of the cell. Here, we investigated the response of a TRP family member, TRPC5, to mechanical stress. Hypoosmolarity triggers Ca²⁺ influx and cationic conductance through TRPC5. Importantly, for the first time we were able to record the stretch-activated TRPC5 current at single-channel level. The activation threshold for TRPC5 was found to be 240 mOsm for hypoosmotic stress and between −20 and −40 mmHg for pressure applied to membrane patch. In addition, we found that disruption of actin filaments suppresses TRPC5 response to hypoosmotic stress and patch pipette pressure, but does not prevent the activation of TRPC5 by stretch-independent mechanisms, indicating that actin cytoskeleton is an essential transduction component that confers mechanosensitivity to TRPC5. In summary, our findings establish that TRPC5 can be activated at the single-channel level when mechanical stress on the cell reaches a certain threshold.

Modulation of innate and learned sexual behaviors by the TRP channel Painless expressed in the fruit fly brain: behavioral genetic analysis and its implications

Transient receptor potential (TRP) channels have attracted considerable attention because of their vital roles in primary sensory neurons, mediating responses to a wide variety of external environmental stimuli. However, much less is known about how TRP channels in the brain respond to intrinsic signals and are involved in neurophysiological processes that control complex behaviors. Painless (Pain) is the Drosophila TRP channel that was initially identified as a molecular sensor responsible for detecting noxious thermal and mechanical stimuli. Here, we review recent behavioral genetic studies demonstrating that Pain expressed in the brain plays a critical role in both innate and learned aspects of sexual behaviors. Several members of the TRP channel superfamily play evolutionarily conserved roles in sensory neurons as well as in other peripheral tissues. It is thus expected that brain TRP channels in vertebrates and invertebrates would have some common physiological functions. Studies of Pain in the Drosophila brain using a unique combination of genetics and physiological techniques should provide valuable insights into the fundamental principles concerning TRP channels expressed in the vertebrate and invertebrate brains.

Brain transient receptor potential channels and stroke

Transient receptor potential (TRP) channels have been increasingly implicated in the pathological mechanisms of CNS disorders. TRP expression has been detected in neurons, astrocytes, oligodendrocytes, microglia, and ependymal cells as well as in the cerebral vascular endothelium and smooth muscle. In stroke, TRPC3/4/6, TRPM2/4/7, and TRPV1/3/4 channels have been found to participate in ischemia-induced cell death, whereas other TRP channels, in particular those expressed in nonneuronal cells, have been less well studied. This review summarizes the current knowledge on the expression and functions of the TRP channels in various cell types in the brain and our current understanding of TRP channels in stroke pathophysiology. In an aging society, the occurrence of stroke is expected to increase steadily, and there is an urgent requirement to improve the current stroke management strategy. Therefore, elucidating the roles of TRP channels in stroke could shed light on the development of novel therapeutic strategies and ultimately improve stroke outcome.

Canonical transient receptor potential 4 and its small molecule modulators

Canonical transient receptor potential 4 (TRPC4) forms non-selective cation channels that contribute to phospholipase C-dependent Ca(2+) entry into cells following stimulation of G protein coupled receptors and receptor tyrosine kinases. Moreover, the channels are regulated by pertussis toxin-sensitive Gi/o proteins, lipids, and various other signaling mechanisms. TRPC4-containing channels participate in the regulation of a variety of physiological functions, including excitability of both gastrointestinal smooth muscles and brain neurons. This review is to present recent advances in the understanding of physiology and development of small molecular modulators of TRPC4 channels.

Riluzole activates TRPC5 channels independently of PLC activity

BACKGROUND AND PURPOSE

The transient receptor potential channel C5 (TRPC5) is a Ca(2+)-permeable cation channel, which is predominantly expressed in the brain. TRPC5 is activated in a PLC-dependent manner by, as yet, unidentified endogenous messengers. Recently, modulators of TRPC5, like Ca(2+), pH and phospholipids, have been identified. However, the role of TRPC5 in vivo is only poorly understood. Novel specific modulators of TRPC5 might help to elucidate its function.

EXPERIMENTAL APPROACH

Novel modulators of TRPC5 were identified in a compound screening of approved drugs and natural compounds. The potency and selectivity of TRPC5-activating compounds were determined by fluorometric calcium imaging. The biophysical properties of channel activation by these compounds were analysed using electrophysiological measurements.

KEY RESULTS

Riluzole was identified as a novel activator of TRPC5 (EC₅₀ 9.2 ± 0.5 μM) and its mechanism of action was shown to be independent of G protein signalling and PLC activity. Riluzole-induced TRPC5 currents were potentiated by La(3+) and, utilizing TRPC5 mutants that lack La(3+) binding sites, it was confirmed that riluzole and La(3+) activate TRPC5 by different mechanisms. Recordings of excised inside-out patches revealed a relatively direct effect of riluzole on TRPC5.

CONCLUSIONS AND IMPLICATIONS

Riluzole can activate TRPC5 heterologously expressed in HEK293 cells as well as those endogenously expressed in the U-87 glioblastoma cell line. Riluzole does not activate any other member of the TRPC family and could, therefore, despite its action on other ion channels, be a useful pharmacological tool for identifying TRPC5-specific currents in immortalized cell lines or in acutely isolated primary cells.

Clemizole hydrochloride is a novel and potent inhibitor of transient receptor potential channel TRPC5

Canonical transient receptor potential channel 5 (TRPC5) is a nonselective, Ca(2+)-permeable cation channel that belongs to the large family of transient receptor potential channels. It is predominantly found in the central nervous system with a high expression density in the hippocampus, the amygdala, and the frontal cortex. Several studies confirm that TRPC5 channels are implicated in the regulation of neurite length and growth cone morphology. We identified clemizole as a novel inhibitor of TRPC5 channels. Clemizole efficiently blocks TRPC5 currents and Ca(2+) entry in the low micromolar range (IC50 = 1.0-1.3 µM), as determined by fluorometric intracellular free Ca(2+) concentration ([Ca(2+)]i) measurements and patch-clamp recordings. Clemizole blocks TRPC5 currents irrespectively of the mode of activation, for example, stimulation of G protein-coupled receptors, hypo-osmotic buffer conditions, or by the direct activator riluzole. Electrophysiological whole-cell recordings revealed that the block was mostly reversible. Moreover, clemizole was still effective in blocking TRPC5 single channels in excised inside-out membrane patches, hinting to a direct block of TRPC5 by clemizole. Based on fluorometric [Ca(2+)]i measurements, clemizole exhibits a sixfold selectivity for TRPC5 over TRPC4β (IC50 = 6.4 µM), the closest structural relative of TRPC5, and an almost 10-fold selectivity over TRPC3 (IC50 = 9.1 µM) and TRPC6 (IC50 = 11.3 µM). TRPM3 and M8 as well as TRPV1, V2, V3, and V4 channels were only weakly affected by markedly higher clemizole concentrations. Clemizole was not only effective in blocking heterologously expressed TRPC5 homomers but also TRPC1:TRPC5 heteromers as well as native TRPC5-like currents in the U-87 glioblastoma cell line.

In pursuit of small molecule chemistry for calcium-permeable non-selective TRPC channels -- mirage or pot of gold?

The primary purpose of this review is to address the progress towards small molecule modulators of human Transient Receptor Potential Canonical proteins (TRPC1, TRPC3, TRPC4, TRPC5, TRPC6 and TRPC7). These proteins generate channels for calcium and sodium ion entry. They are relevant to many mammalian cell types including acinar gland cells, adipocytes, astrocytes, cardiac myocytes, cochlea hair cells, endothelial cells, epithelial cells, fibroblasts, hepatocytes, keratinocytes, leukocytes, mast cells, mesangial cells, neurones, osteoblasts, osteoclasts, platelets, podocytes, smooth muscle cells, skeletal muscle and tumour cells. There are broad-ranging positive roles of the channels in cell adhesion, migration, proliferation, survival and turning, vascular permeability, hypertrophy, wound-healing, hypo-adiponectinaemia, angiogenesis, neointimal hyperplasia, oedema, thrombosis, muscle endurance, lung hyper-responsiveness, glomerular filtration, gastrointestinal motility, pancreatitis, seizure, innate fear, motor coordination, saliva secretion, mast cell degranulation, cancer cell drug resistance, survival after myocardial infarction, efferocytosis, hypo-matrix metalloproteinase, vasoconstriction and vasodilatation. Known small molecule stimulators of the channels include hyperforin, genistein and rosiglitazone, but there is more progress with inhibitors, some of which have promising potency and selectivity. The inhibitors include 2-aminoethoxydiphenyl borate, 2-aminoquinolines, 2-aminothiazoles, fatty acids, isothiourea derivatives, naphthalene sulfonamides, N-phenylanthranilic acids, phenylethylimidazoles, piperazine/piperidine analogues, polyphenols, pyrazoles and steroids. A few of these agents are starting to be useful as tools for determining the physiological and pathophysiological functions of TRPC channels. We suggest that the pursuit of small molecule modulators for TRPC channels is important but that it requires substantial additional effort and investment before we can reap the rewards of highly potent and selective pharmacological modulators.

Physiological Function and Characterization of TRPCs in Neurons

Ca²⁺ entry is essential for regulating vital physiological functions in all neuronal cells. Although neurons are engaged in multiple modes of Ca²⁺ entry that regulates variety of neuronal functions, we will only discuss a subset of specialized Ca²⁺-permeable non-selective Transient Receptor Potential Canonical (TRPC) channels and summarize their physiological and pathological role in these excitable cells. Depletion of endoplasmic reticulum (ER) Ca²⁺ stores, due to G-protein coupled receptor activation, has been shown to activate TRPC channels in both excitable and non-excitable cells. While all seven members of TRPC channels are predominately expressed in neuronal cells, the ion channel properties, mode of activation, and their physiological responses are quite distinct. Moreover, many of these TRPC channels have also been suggested to be associated with neuronal development, proliferation and differentiation. In addition, TRPCs also regulate neurosecretion, long-term potentiation and synaptic plasticity. Similarly, perturbations in Ca²⁺ entry via the TRPC channels have been also suggested in a spectrum of neuropathological conditions. Hence, understanding the precise involvement of TRPCs in neuronal function and in neurodegenerative conditions would presumably unveil avenues for plausible therapeutic interventions for these devastating neuronal diseases.