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

Transient receptor potential canonical 4 (TRPC4) is a receptor-operated cation channel codependent on both the Gq/11­phospholipase C signaling pathway and Gi/o proteins for activation. This makes TRPC4 an excellent coincidence sensor of neurotransmission through Gq/11- and Gi/o-coupled receptors. In whole-cell slice recordings of lateral septal neurons, TRPC4 mediates a strong depolarizing plateau that shuts down action potential firing, which may or may not be followed by a hyperpolarization that extends the firing pause to varying durations depending on the strength of Gi/o stimulation. We show that the depolarizing plateau is codependent on Gq/11-coupled group I metabotropic glutamate receptors and on Gi/o-coupled γ-aminobutyric acid type B receptors. The hyperpolarization is mediated by Gi/o activation of G protein­activated inwardly rectifying K+ (GIRK) channels. Moreover, the firing patterns, elicited by either electrical stimulation or receptor agonists, encode information about the relative strengths of Gq/11 and Gi/o inputs in the following fashion. Pure Gq/11 input produces weak depolarization accompanied by firing acceleration, whereas pure Gi/o input causes hyperpolarization that pauses firing. Although coincident Gq/11­Gi/o inputs also pause firing, the pause is preceded by a burst, and both the pause duration and firing recovery patterns reflect the relative strengths of Gq/11 versus Gi/o inputs. Computer simulations demonstrate that different combinations of TRPC4 and GIRK conductances are sufficient to produce the range of firing patterns observed experimentally. Thus, concurrent neurotransmission through the Gq/11 and Gi/o pathways is converted to discernible electrical responses by the joint actions of TRPC4 and GIRK for communication to downstream neurons.

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

KEY POINTS: Receptor-operated activation of TRPC4 cation channels requires Gi/o proteins and phospholipase-Cδ1 (PLCδ1) activation by intracellular Ca2+ . Concurrent stimulation of the Gq/11 pathway accelerates Gi/o activation of TRPC4, which is not mimicked by increasing cytosolic Ca2+ . The kinetic effect of Gq/11 was diminished by alkaline intracellular pH (pHi ) and increased pHi buffer capacity. Acidic pHi (6.75-6.25) together with the cytosolic Ca2+ rise accelerated Gi/o -mediated TRPC4 activation. Protons exert their facilitation effect through Ca2+ -dependent activation of PLCδ1. The data suggest that the Gq/11 -PLCß pathway facilitates Gi/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 Gq/11 and Gi/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 Gi/o proteins, with a co-dependence on phospholipase-Cδ1 (PLCδ1). The Gi/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 TRPC4ß and the Gi/o -coupled µ opioid receptor, we investigated the role of intracellular protons on Gi/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 Gi/o and Gq/11 co-stimulation induces a faster activation of TRPC4 than Gi/o activation alone and highlights again the critical role of PLCδ1 in TRPC4 gating.

Critical roles of Gi/o proteins and phospholipase C-δ1 in the activation of receptor-operated TRPC4 channels.

Transient Receptor Potential Canonical (TRPC) proteins form nonselective cation channels commonly known to be activated downstream from receptors that signal through phospholipase C (PLC). Although TRPC3/C6/C7 can be directly activated by diacylglycerols produced by PLC breakdown of phosphatidylinositol 4,5-bisphosphate (PIP2), the mechanism by which the PLC pathway activates TRPC4/C5 remains unclear. We show here that TRPC4 activation requires coincident stimulation of Gi/o subgroup of G proteins and PLCδ, with a preference for PLCδ1 over PLCδ3, but not necessarily the PLCß pathway commonly thought to be involved in receptor-operated TRPC activation. In HEK293 cells coexpressing TRPC4 and Gi/o-coupled µ opioid receptor, µ agonist elicited currents biphasically, with an initial slow phase preceding a rapidly developing phase. The currents were dependent on intracellular Ca(2+) and PIP2. Reducing PIP2 through phosphatases abolished the biphasic kinetics and increased the probability of channel activation by weak Gi/o stimulation. In both HEK293 cells heterologously expressing TRPC4 and renal carcinoma-derived A-498 cells endogenously expressing TRPC4, channel activation was inhibited by knocking down PLCδ1 levels and almost completely eliminated by a dominant-negative PLCδ1 mutant and a constitutively active RhoA mutant. Conversely, the slow phase of Gi/o-mediated TRPC4 activation was diminished by inhibiting RhoA or enhancing PLCδ function. Our data reveal an integrative mechanism of TRPC4 on detection of coincident Gi/o, Ca(2+), and PLC signaling, which is further modulated by the small GTPase RhoA. This mechanism is not shared with the closely related TRPC5, implicating unique roles of TRPC4 in signal integration in brain and other systems.

Dynamic coupling between TRPV4 and Ca2+-activated SK1/3 and IK1 K+ channels plays a critical role in regulating the K+-secretory BK channel in kidney collecting duct cells.

The large-conductance Ca2+-activated K+ channel, BK (KCNMA1), is expressed along the connecting tubule (CNT) and cortical collecting duct (CCD) where it underlies flow- and Ca2+-dependent K+ secretion. Its activity is partially under the control of the mechanosensitive transient receptor potential vanilloid type 4 (TRPV4) Ca2+-permeable channel. Recently, we identified three small-/intermediate-conductance Ca2+-activated K+ channels, SK1 (KCNN1), SK3 (KCNN3), and IK1 (KCNN4), with notably high Ca2+-binding affinities, that are expressed in CNT/CCD and may be regulated by TRPV4-mediated Ca2+ influx. The K+-secreting CCD mCCDcl1 cells, which express these channels, were used to determine whether SK1/3 and IK1 are activated on TRPV4 stimulation and whether they contribute to Ca2+ influx and activation of BK. Activation of TRPV4 (GSK1016790A) modestly depolarized the membrane potential and robustly increased intracellular Ca2+, [Ca2+]i Inhibition of both SK1/3 and IK1 by application of apamin and 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34), respectively, further depolarized the membrane potential and markedly suppressed the TRPV4-mediated rise in [Ca2+]i Application of BK inhibitor iberiotoxin after activation of TRPV4 without apamin/TRAM-34 also reduced [Ca2+]i and further intensified membrane depolarization, demonstrating BK involvement. However, the BK-dependent effects on [Ca2+]i and membrane potential were largely abolished by pretreatment with apamin and TRAM-34, identical to that observed by separately suppressing TRPV4-mediated Ca2+ influx, demonstrating that SK1/3-IK1 channels potently contribute to TRPV4-mediated BK activation. Our data indicate a direct correlation between TRPV4-mediated Ca2+ signal and BK activation but where early activation of SK1/3 and IK1 channels are critical to sufficiently enhanced Ca2+ entry and [Ca2+]i levels required for activation of BK.

Regulator of G-protein signalling and GoLoco proteins suppress TRPC4 channel function via acting at Gαi/o.

Transient receptor potential canonical 4 (TRPC4) forms non-selective cation channels implicated in the regulation of diverse physiological functions. Previously, TRPC4 was shown to be activated by the Gi/o subgroup of heterotrimeric G-proteins involving Gαi/o, rather than Gßγ, subunits. Because the lifetime and availability of Gα-GTP are regulated by regulators of G-protein signalling (RGS) and Gαi/o-Loco (GoLoco) domain-containing proteins via their GTPase-activating protein (GAP) and guanine-nucleotide-dissociation inhibitor (GDI) functions respectively, we tested how RGS and GoLoco domain proteins affect TRPC4 currents activated via Gi/o-coupled receptors. Using whole-cell patch-clamp recordings, we show that both RGS and GoLoco proteins [RGS4, RGS6, RGS12, RGS14, LGN or activator of G-protein signalling 3 (AGS3)] suppress receptor-mediated TRPC4 activation without causing detectable basal current or altering surface expression of the channel protein. The inhibitory effects are dependent on the GAP and GoLoco domains and facilitated by enhancing membrane targeting of the GoLoco protein AGS3. In addition, RGS, but not GoLoco, proteins accelerate desensitization of receptor-activation evoked TRPC4 currents. The inhibitory effects of RGS and GoLoco domains are additive and are most prominent with RGS12 and RGS14, which contain both RGS and GoLoco domains. Our data support the notion that the Gα, but not Gßγ, arm of the Gi/o signalling is involved in TRPC4 activation and unveil new roles for RGS and GoLoco domain proteins in fine-tuning TRPC4 activities. The versatile and diverse functions of RGS and GoLoco proteins in regulating G-protein signalling may underlie the complexity of receptor-operated TRPC4 activation in various cell types under different conditions.

New Aspects of the Contribution of ER to SOCE Regulation: TRPC Proteins as a Link Between Plasma Membrane Ion Transport and Intracellular Ca2+ Stores.

Transient receptor potential canonical (TRPC) proteins were identified as molecular candidates of receptor- and/or store-operated channels because of their close homology to the Drosophila TRP and TRPL. Functional studies have revealed that TRPC channels play an integrated part of phospholipase C-transduced cell signaling, mediating the influx of both Ca2+ and Na+ into cells. As a consequence, the TRPC channels have diverse functional roles in different cell types, including metabotropic receptor-evoked membrane depolarization and intracellular Ca2+ concentration elevation. Depending on the cellular environment and the protein partners present in the channel complex, the TRPC channels display different biophysical properties and mechanisms of regulation, including but not limited to the Ca2+ filling state of the endoplasmic reticulum. Despite the overwhelming focus on STIM-regulated Orai channels for store-operated Ca2+ entry, evidence is growing for STIM-operated TRPC channel activities in various cell types, demonstrating both store-dependent and store-independent mechanisms of TRPC channel gating. The existence of physical and functional interactions between plasma membrane-localized TRPC channels and other proteins involved in sensing and regulating the intracellular Ca2+ store contents, such as inositol trisphosphate receptors, Junctate, and Homer, further argues for the role of TRPC proteins in linking plasma membrane ion transport with intracellular Ca2+ stores. The interplay among these proteins will likely define the functional significance of TRPC channel activation in different cellular contexts and under different modes of stimulations.

Posttraumatic stress disorder-like induction elevates ß-amyloid levels, which directly activates corticotropin-releasing factor neurons to exacerbate stress responses.

Recent studies have found that those who suffer from posttraumatic stress disorder (PTSD) are more likely to experience dementia as they age, most often Alzheimer's disease (AD). These findings suggest that the symptoms of PTSD might have an exacerbating effect on AD progression. AD and PTSD might also share common susceptibility factors such that those who experience trauma-induced disease were already more likely to succumb to dementia with age. Here, we explored these two hypotheses using a mouse model of PTSD in wild-type and AD model animals. We found that expression of human familial AD mutations in amyloid precursor protein and presenilin 1 leads to sensitivity to trauma-induced PTSD-like changes in behavioral and endocrine stress responses. PTSD-like induction, in turn, chronically elevates levels of CSF ß-amyloid (Aß), exacerbating ongoing AD pathogenesis. We show that PTSD-like induction and Aß elevation are dependent on corticotropin-releasing factor (CRF) receptor 1 signaling and an intact hypothalamic-pituitary-adrenal axis. Furthermore, we show that Aß species can hyperexcite CRF neurons, providing a mechanism by which Aß influences stress-related symptoms and PTSD-like phenotypes. Consistent with Aß causing excitability of the stress circuitry, we attenuate PTSD-like phenotypes in vivo by lowering Aß levels during PTSD-like trauma exposure. Together, these data demonstrate that exposure to PTSD-like trauma can drive AD pathogenesis, which directly perturbs CRF signaling, thereby enhancing chronic PTSD symptoms while increasing risk for AD-related dementia.

p75 regulates Purkinje cell firing by modulating SK channel activity through Rac1.

p75 is expressed among Purkinje cells in the adult cerebellum, but its function has remained obscure. Here we report that p75 is involved in maintaining the frequency and regularity of spontaneous firing of Purkinje cells. The overall spontaneous firing activity of Purkinje cells was increased in p75(-/-) mice during the phasic firing period due to a longer firing period and accompanying reduction in silence period than in the wild type. We attribute these effects to a reduction in small conductance Ca(2+)-activated potassium (SK) channel activity in Purkinje cells from p75(-/-) mice compared with the wild type littermates. The mechanism by which p75 regulates SK channel activity appears to involve its ability to activate Rac1. In organotypic cultures of cerebellar slices, brain-derived neurotrophic factor increased RacGTP levels by activating p75 but not TrkB. These results correlate with a reduction in RacGTP levels in synaptosome fractions from the p75(-/-) cerebellum, but not in that from the cortex of the same animals, compared with wild type littermates. More importantly, we demonstrate that Rac1 modulates SK channel activity and firing patterns of Purkinje cells. Along with the finding that spine density was reduced in p75(-/-) cerebellum, these data suggest that p75 plays a role in maintaining normalcy of Purkinje cell firing in the cerebellum in part by activating Rac1 in synaptic compartments and modulating SK channels.

Dual depolarization responses generated within the same lateral septal neurons by TRPC4-containing channels.

In the central nervous system, canonical transient receptor potential (TRPC) channels have been implicated in mediating neuronal excitation induced by stimulating metabotropic receptors, including group 1 metabotropic glutamate receptors (mGluRs). Lateral septal (LS) neurons express high levels of TRPC4 and group I mGluRs. However, to what extent native TRPC4-containing channels (TRPC4-cc) are activated as well as the impact of different levels of TRPC4-cc activation on neuronal excitability remain elusive. Here, we report that stimulating LS neurons with group I mGluR agonist, (S)-3,5-DHPG, causes either an immediate increase in firing rate or an initial burst followed by a pause of firing, which can be correlated with below-threshold-depolarization (BTD) or above-threshold-plateau-depolarization (ATPD), respectively, in whole-cell recordings. The early phase of BTD and the entire ATPD are completely absent in neurons from TRPC4−/− mice. Moreover, in the same LS neurons, BTD can be converted to ATPD at more depolarized potentials or with a brief current injection, suggesting that BTD and ATPD may represent partial and full activations of TRPC4-cc, respectively. We show that coincident mGluR stimulation and depolarization is required to evoke strong TRPC4-cc current, and Na+ and Ca2+ influx, together with dynamic changes of intracellular Ca(2+), are essential for ATPD induction. Our results suggest that TRPC4-cc integrates metabotropic receptor stimulation with intracellular Ca(2+) signals to generate two interconvertible depolarization responses to affect excitability of LS neurons in distinct fashions.

Readiness of nociceptor cell bodies to generate spontaneous activity results from background activity of diverse ion channels and high input resistance.

ABSTRACT: Nociceptor cell bodies generate "spontaneous" discharge that can promote ongoing pain in persistent pain conditions. Little is known about the underlying mechanisms. Recordings from nociceptor cell bodies (somata) dissociated from rodent and human dorsal root ganglia have shown that previous pain in vivo is associated with low-frequency discharge controlled by irregular depolarizing spontaneous fluctuations of membrane potential (DSFs), likely produced by transient inward currents across the somal input resistance. Using mouse nociceptors, we show that DSFs are associated with high somal input resistance over a wide range of membrane potentials, including depolarized levels where DSFs approach action potential (AP) threshold. Input resistance and both the amplitude and frequency of DSFs were increased in neurons exhibiting spontaneous activity. Ion substitution experiments indicated that the depolarizing phase of DSFs is generated by spontaneous opening of channels permeable to Na + or Ca 2+ and that Ca 2+ -permeable channels are especially important for larger DSFs. Partial reduction of the amplitude or frequency of DSFs by perfusion of pharmacological inhibitors indicated small but significant contributions from Nav1.7, Nav1.8, TRPV1, TRPA1, TRPM4, and N-type Ca 2+ channels. Less specific blockers suggested a contribution from NALCN channels, and global knockout suggested a role for Nav1.9. The combination of high somal input resistance plus background activity of diverse ion channels permeable to Na + or Ca 2+ produces DSFs that are poised to reach AP threshold if resting membrane potential depolarizes, AP threshold decreases, or DSFs become enhanced-all of which can occur under painful neuropathic and inflammatory conditions.

Widespread latent hyperactivity of nociceptors outlasts enhanced avoidance behavior following incision injury.

Nociceptors with somata in dorsal root ganglia (DRGs) exhibit an unusual readiness to switch from an electrically silent state to a hyperactive state of tonic, nonaccommodating, low-frequency, irregular discharge of action potentials (APs). Ongoing activity (OA) during this state is present in vivo in rats months after spinal cord injury (SCI), and has been causally linked to SCI pain. OA induced by various neuropathic conditions in rats, mice, and humans is retained in nociceptor somata after dissociation and culturing, providing a powerful tool for investigating its mechanisms and functions. An important question is whether similar nociceptor OA is induced by painful conditions other than neuropathy. The present study shows that probable nociceptors dissociated from DRGs of rats subjected to postsurgical pain (induced by plantar incision) exhibit OA. The OA was most apparent when the soma was artificially depolarized to a level within the normal range of membrane potentials where large, transient depolarizing spontaneous fluctuations (DSFs) can approach AP threshold. This latent hyperactivity persisted for at least 3 weeks, whereas behavioral indicators of affective pain - hindpaw guarding and increased avoidance of a noxious substrate in an operant conflict test - persisted for 1 week or less. An unexpected discovery was latent OA in neurons from thoracic DRGs that innervate dermatomes distant from the injured tissue. The most consistent electrophysiological alteration associated with OA was enhancement of DSFs. Potential in vivo functions of widespread, low-frequency nociceptor OA consistent with these and other findings are to amplify hyperalgesic priming and to drive anxiety-related hypervigilance.

Changes in Spontaneous firing patterns of cerebellar Purkinje cells in p75 knockout mice.

The p75 neurotrophin receptor is highly expressed in the developing nervous system and is required for neuronal survival, growth, and synaptic transmission. In young mice, p75 is present in both granular cells and Purkinje cells of the cerebellum. Although p75 has been implicated in modulation of neuronal excitability in several neuronal types, whether and how it affects the excitability of cerebellar Purkinje neurons remained unclear. Using extracellular recordings of spontaneous firing of Purkinje neurons in cerebellar slices prepared from wild type and p75 knockout mice, we measured intrinsic firing properties in the presence of fast synaptic blockers of more than 200 Purkinje cells, each for a period of 5 min, for each genotype. We detected a significant increase in the mean firing frequency in p75(-/-) neurons comparing to the wild type littermates. Upon separating tonically firing from phasically firing cells, i.e., cells with firing pauses of longer than 300 ms, we observed that the change mainly arose from phasic firing cells and can be explained by an increase in the firing/silence ratio and a decrease in the number of long pauses during the 5-min recording period. We conclude that p75 plays an important role in regulating the firing-to-silence transition during the phasic firing period of the spontaneous firing of Purkinje cells. Thus, p75 exerts a modulatory function on Purkinje cell firing patterns, through which it may act as a key player in motor coordination and other cerebellum-regulated activities since Purkinje cells represent the sole neuronal output of the cerebellar cortex.

Identification of ML204, a novel potent antagonist that selectively modulates native TRPC4/C5 ion channels.

Transient receptor potential canonical (TRPC) channels are Ca(2+)-permeable nonselective cation channels implicated in diverse physiological functions, including smooth muscle contractility and synaptic transmission. However, lack of potent selective pharmacological inhibitors for TRPC channels has limited delineation of the roles of these channels in physiological systems. Here we report the identification and characterization of ML204 as a novel, potent, and selective TRPC4 channel inhibitor. A high throughput fluorescent screen of 305,000 compounds of the Molecular Libraries Small Molecule Repository was performed for inhibitors that blocked intracellular Ca(2+) rise in response to stimulation of mouse TRPC4ß by µ-opioid receptors. ML204 inhibited TRPC4ß-mediated intracellular Ca(2+) rise with an IC(50) value of 0.96 µm and exhibited 19-fold selectivity against muscarinic receptor-coupled TRPC6 channel activation. In whole-cell patch clamp recordings, ML204 blocked TRPC4ß currents activated through either µ-opioid receptor stimulation or intracellular dialysis of guanosine 5'-3-O-(thio)triphosphate (GTPγS), suggesting a direct interaction of ML204 with TRPC4 channels rather than any interference with the signal transduction pathways. Selectivity studies showed no appreciable block by 10-20 µm ML204 of TRPV1, TRPV3, TRPA1, and TRPM8, as well as KCNQ2 and native voltage-gated sodium, potassium, and calcium channels in mouse dorsal root ganglion neurons. In isolated guinea pig ileal myocytes, ML204 blocked muscarinic cation currents activated by bath application of carbachol or intracellular infusion of GTPγS, demonstrating its effectiveness on native TRPC4 currents. Therefore, ML204 represents an excellent novel tool for investigation of TRPC4 channel function and may facilitate the development of therapeutics targeted to TRPC4.

Rapid and efficient generation of functional motor neurons from human pluripotent stem cells using gene delivered transcription factor codes.

Stem cell-derived motor neurons (MNs) are increasingly utilized for modeling disease in vitro and for developing cellular replacement strategies for spinal cord injury and diseases such as spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS). Human embryonic stem cell (hESC) differentiation into MNs, which involves retinoic acid (RA) and activation of the sonic hedgehog (SHH) pathway is inefficient and requires up to 60 days to develop MNs with electrophysiological properties. This prolonged differentiation process has hampered the use of hESCs, in particular for high-throughput screening. We evaluated the MN gene expression profile of RA/SHH-differentiated hESCs to identify rate-limiting factors involved in MN development. Based on this analysis, we developed an adenoviral gene delivery system encoding for MN inducing transcription factors: neurogenin 2 (Ngn2), islet-1 (Isl-1), and LIM/homeobox protein 3 (Lhx3). Strikingly, delivery of these factors induced functional MNs with mature electrophysiological properties, 11-days after gene delivery, with >60-70% efficiency from hESCs and human induced pluripotent stem cells (hiPSCs). This directed programming approach significantly reduces the time required to generate electrophysiologically-active MNs by approximately 30 days in comparison to conventional differentiation techniques. Our results further exemplify the potential to use transcriptional coding for rapid and efficient production of defined cell types from hESCs and hiPSCs.

The Three Two-Pore Channel Subtypes from Rabbit Exhibit Distinct Sensitivity to Phosphoinositides, Voltage, and Extracytosolic pH.

Two pore channels (TPCs) are implicated in vesicle trafficking, virus infection, and autophagy regulation. As Na+- or Ca2+-permeable channels, TPCs have been reported to be activated by NAADP, PI(3,5)P2, and/or high voltage. However, a comparative study on the function and regulation of the three mammalian TPC subtypes is currently lacking. Here, we used the electrophysiological recording of enlarged endolysosome vacuoles, inside-out and outside-out membrane patches to examine the three TPCs of rabbit (Oryctolagus cuniculus, or Oc) heterologously expressed in HEK293 cells. While PI(3,5)P2 evoked Na+ currents with a potency order of OcTPC1 > OcTPC3 > OcTPC2, only OcTPC2 displayed a strict dependence on PI(3,5)P2. Both OcTPC1 and OcTPC3 were activatable by PI3P and OcTPC3 was also activated by additional phosphoinositide species. While OcTPC2 was voltage-independent, OcTPC1 and OcTPC3 showed voltage dependence with OcTPC3 depending on high positive voltages. Finally, while OcTPC2 preferred a luminal pH of 4.6−6.0 in endolysosomes, OcTPC1 was strongly inhibited by extracytosolic pH 5.0 in both voltage-dependent and -independent manners, and OcTPC3 was inhibited by pH 6.0 but potentiated by pH 8.0. Thus, the three OcTPCs form phosphoinositide-activated Na+ channels with different ligand selectivity, voltage dependence, and extracytosolic pH sensitivity, which likely are optimally tuned for function in specific endolysosomal populations.

Mechanisms of proton inhibition and sensitization of the cation channel TRPV3.

TRPV3 is a temperature-sensitive, nonselective cation channel expressed prominently in skin keratinocytes. TRPV3 plays important roles in hair morphogenesis and maintenance of epidermal barrier function. Gain-of-function mutations of TRPV3 have been found in both humans and rodents and are associated with hair loss, pruritus, and dermatitis. Here, we study the mechanisms of acid regulation of TRPV3 by using site-directed mutagenesis, fluorescent intracellular calcium measurement, and whole-cell patch-clamp recording techniques. We show that, whereas extracellular acid inhibits agonist-induced TRPV3 activation through an aspartate residue (D641) in the selectivity filter, intracellular protons sensitize the channel through cytoplasmic C-terminal glutamate and aspartate residues (E682, E689, and D727). Neutralization of the three C-terminal residues presensitizes the channel to agonist stimulation. Molecular dynamic simulations revealed that charge neutralization of the three C-terminal residues stabilized the sensitized channel conformation and enhanced the probability of α-helix formation in the linker between the S6 transmembrane segment and TRP domain. We conclude that acid inhibits TRPV3 function from the extracellular side but facilitates it from the intracellular side. These novel mechanisms of TRPV3 proton sensing can offer new insights into the role of TRPV3 in the regulation of epidermal barrier permeability and skin disorders under conditions of tissue acidosis.

THE CONCISE GUIDE TO PHARMACOLOGY 2021/22: Ion channels.

The Concise Guide to PHARMACOLOGY 2021/22 is the fifth in this series of biennial publications. The Concise Guide provides concise overviews, mostly in tabular format, of the key properties of nearly 1900 human drug targets with an emphasis on selective pharmacology (where available), plus links to the open access knowledgebase source of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. Although the Concise Guide constitutes over 500 pages, the material presented is substantially reduced compared to information and links presented on the website. It provides a permanent, citable, point-in-time record that will survive database updates. The full contents of this section can be found at http://onlinelibrary.wiley.com/doi/bph.15539. Ion channels are one of the six major pharmacological targets into which the Guide is divided, with the others being: G protein-coupled receptors, nuclear hormone receptors, catalytic receptors, enzymes and transporters. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. The landscape format of the Concise Guide is designed to facilitate comparison of related targets from material contemporary to mid-2021, and supersedes data presented in the 2019/20, 2017/18, 2015/16 and 2013/14 Concise Guides and previous Guides to Receptors and Channels. It is produced in close conjunction with the Nomenclature and Standards Committee of the International Union of Basic and Clinical Pharmacology (NC-IUPHAR), therefore, providing official IUPHAR classification and nomenclature for human drug targets, where appropriate.

Activation of TRPA1 channels by fenamate nonsteroidal anti-inflammatory drugs.

Transient receptor potential A1 (TRPA1) forms nonselective cation channels implicated in acute inflammatory pain and nociception. The mechanism of ligand activation of TRPA1 may involve either covalent modification of cysteine residues or conventional reversible ligand-receptor interactions. For certain electrophilic prostaglandins, covalent modification has been considered as the main mechanism involved in their stimulatory effect on TRPA1. Because some nonsteroidal anti-inflammatory drugs (NSAIDs) are structural analogs of prostaglandins, we examined several nonelectrophilic NSAIDs on TRPA1 activation using electrophysiological techniques and intracellular Ca(2+) measurements and found that a selected group of NSAIDs can act as TRPA1 agonists. Extracellularly applied flufenamic, niflumic, and mefenamic acid, as well as flurbiprofen, ketoprofen, diclofenac, and indomethacin, rapidly activated rat TRPA1 expressed in Xenopus oocytes and human TRPA1 endogenously expressed in WI-38 fibroblasts. Similarly, the NSAID ligands activated human TRPA1 inducibly expressed in HEK293 cells, but the responses were absent in uninduced and parental HEK293 cells. The response to fenamate agonists was blocked by TRPA1 antagonists, AP-18, HC-030031, and ruthenium red. At subsaturating concentrations, the fenamate NSAIDs also potentiate the activation of TRPA1 by allyl isothiocyanate, cinnamaldehyde, and cold, demonstrating positive synergistic interactions with other well-characterized TRPA1 activators. Importantly, among several thermosensitive TRP channels, the stimulatory effect is specific to TRPA1 because flufenamic acid inhibited TRPV1, TRPV3, and TRPM8. We conclude that fenamate NSAIDs are a novel class of potent and reversible direct agonists of TRPA1. This selective group of TRPA1-stimulating NSAIDs should provide a structural basis for developing novel ligands that noncovalently interact with TRPA1 channels.

Sensorimotor enhancement in mouse mutants lacking the Purkinje cell-specific Gi/o modulator, Pcp2(L7).

Pcp2(L7) is a GoLoco domain protein specifically and abundantly expressed in cerebellar Purkinje cells. It has been hypothesized to "tune" G(i/o)-coupled receptor modulation of physiological effectors, including the P-type Ca(2+) channel. We have analyzed a mouse mutant in which the Pcp2(L7) gene was inactivated and find significant anatomical, behavioral and electrophysiological changes. Anatomically, we observed mild cerebellar hypoplasia. Behaviorally, the mutants were altered in modalities atypical for a traditional cerebellar mutant, and oddly, all of these changes could be considered functional enhancements. This includes increased asymptotic performance in gross motor learning, increased rate of acquisition in tone-conditioned fear, and enhanced pre-pulse inhibition of the acoustic startle response. Electrophysiological analysis of Purkinje cells in the mutants reveals depression of the complex spike waveform that may underlie the behavioral changes. Based on these observations we suggest that the Pcp2(L7) protein acts as a sensorimotor damper that modulates time- and sense-dependent changes in motor responses.

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 Gq/11-coupled receptors, TRPC4 requires simultaneous stimulation of Gi/o-coupled receptors, making it a perfect detector of coincident Gi/o and Gq/11 signaling. Evidence shows that activated Gαi/o proteins work together with PLCδ1 to induce robust TRPC4 activation and the process is accelerated by stimulation of other PLC isozymes, such as PLCß through Gq/11 proteins. Mechanistically, Gq/11-PLCß activation produces triggering proton and calcium signals to initiate self-propagating PLCδ1 activity, crucial for Gi/o-mediated TRPC4 function. Thus, TRPC4-containing channels are activated under conditions not only when coincident Gi/o and Gq/11 stimulation occurs, but also when Gi/o stimulation coincides with proton and Ca2+ signals. The resulting cytosolic Ca2+ rise and membrane depolarization switch the inhibitory Gi/o response to excitation. The conditions and implications of Gi/o-mediated TRPC4 activation in physiology and pathophysiology warrant further investigation.