A diacylglycerol-activated Ca2+ channel in PC12 cells (an adrenal chromaffin cell line) correlates with expression of the TRP-6 (transient receptor potential) protein

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

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

A sodium background conductance controls the spiking pattern of mouse adrenal chromaffin cells in situ

KEY POINTS

Mouse chromaffin cells in acute adrenal slices exhibit two distinct spiking patterns, a repetitive mode and a bursting mode. A sodium background conductance operates at rest as demonstrated by the membrane hyperpolarization evoked by a low Na⁺ -containing extracellular saline. This sodium background current is insensitive to TTX, is not blocked by Cs⁺ ions and displays a linear I-V relationship at potentials close to chromaffin cell resting potential. Its properties are reminiscent of those of the sodium leak channel NALCN. In the adrenal gland, Nalcn mRNA is selectively expressed in chromaffin cells. The study fosters our understanding of how the spiking pattern of chromaffin cells is regulated and adds a sodium background conductance to the list of players involved in the stimulus-secretion coupling of the adrenomedullary tissue.

ABSTRACT

Chromaffin cells (CCs) are the master neuroendocrine units for the secretory function of the adrenal medulla and a finely-tuned regulation of their electrical activity is required for appropriate catecholamine secretion in response to the organismal demand. Here, we aim at deciphering how the spiking pattern of mouse CCs is regulated by the ion conductances operating near the resting membrane potential (RMP). At RMP, mouse CCs display a composite firing pattern, alternating between active periods composed of action potentials spiking with a regular or a bursting mode, and silent periods. RMP is sensitive to changes in extracellular sodium concentration, and a low Na⁺ -containing saline hyperpolarizes the membrane, regardless of the discharge pattern. This RMP drive reflects the contribution of a depolarizing conductance, which is (i) not blocked by tetrodotoxin or caesium, (ii) displays a linear I-V relationship between -110 and -40 mV, and (iii) is carried by cations with a conductance sequence g_Na  > g_K  > g_Cs . These biophysical attributes, together with the expression of the sodium-leak channel Nalcn transcript in CCs, state credible the contribution of NALCN. This inaugural report opens new research routes in the field of CC stimulus-secretion coupling, and extends the inventory of tissues in which NALCN is expressed to neuroendocrine glands.

Potential Drug Candidates to Treat TRPC6 Channel Deficiencies in the Pathophysiology of Alzheimer's Disease and Brain Ischemia

Alzheimer’s disease and cerebral ischemia are among the many causative neurodegenerative diseases that lead to disabilities in the middle-aged and elderly population. There are no effective disease-preventing therapies for these pathologies. Recent in vitro and in vivo studies have revealed the TRPC6 channel to be a promising molecular target for the development of neuroprotective agents. TRPC6 channel is a non-selective cation plasma membrane channel that is permeable to Ca²⁺. Its Ca²⁺-dependent pharmacological effect is associated with the stabilization and protection of excitatory synapses. Downregulation as well as upregulation of TRPC6 channel functions have been observed in Alzheimer’s disease and brain ischemia models. Thus, in order to protect neurons from Alzheimer’s disease and cerebral ischemia, proper TRPC6 channels modulators have to be used. TRPC6 channels modulators are an emerging research field. New chemical structures modulating the activity of TRPC6 channels are being currently discovered. The recent publication of the cryo-EM structure of TRPC6 channels should speed up the discovery process even more. This review summarizes the currently available information about potential drug candidates that may be used as basic structures to develop selective, highly potent TRPC6 channel modulators to treat neurodegenerative disorders, such as Alzheimer’s disease and cerebral ischemia.

Direct Activation of TRPC3 Channels by the Antimalarial Agent Artemisinin

(1) Background: Members of the TRPC3/TRPC6/TRPC7 subfamily of canonical transient receptor potential (TRP) channels share an amino acid similarity of more than 80% and can form heteromeric channel complexes. They are directly gated by diacylglycerols in a protein kinase C-independent manner. To assess TRPC3 channel functions without concomitant protein kinase C activation, direct activators are highly desirable. (2) Methods: By screening 2000 bioactive compounds in a Ca²⁺ influx assay, we identified artemisinin as a TRPC3 activator. Validation and characterization of the hit was performed by applying fluorometric Ca²⁺ influx assays and electrophysiological patch-clamp experiments in heterologously or endogenously TRPC3-expressing cells. (3) Results: Artemisinin elicited Ca²⁺ entry through TRPC3 or heteromeric TRPC3:TRPC6 channels, but did not or only weakly activated TRPC6 and TRPC7. Electrophysiological recordings confirmed the reversible and repeatable TRPC3 activation by artemisinin that was inhibited by established TRPC3 channel blockers. Rectification properties and reversal potentials were similar to those observed after stimulation with a diacylglycerol mimic, indicating that artemisinin induces a similar active state as the physiological activator. In rat pheochromocytoma PC12 cells that endogenously express TRPC3, artemisinin induced a Ca²⁺ influx and TRPC3-like currents. (4) Conclusions: Our findings identify artemisinin as a new biologically active entity to activate recombinant or native TRPC3-bearing channel complexes in a membrane-confined fashion.

Permeation, regulation and control of expression of TRP channels by trace metal ions

Transient receptor potential (TRP) channels form a diverse family of cation channels comprising 28 members in mammals. Although some TRP proteins can only be found on intracellular membranes, most of the TRP protein isoforms reach the plasma membrane where they form ion channels and control a wide number of biological processes. There, their involvement in the transport of cations such as calcium and sodium has been well documented. However, a growing number of studies have started to expand our understanding of these proteins by showing that they also transport other biologically relevant metal ions like zinc, magnesium, manganese and cobalt. In addition to this newly recognized property, the activity and expression of TRP channels can be regulated by metal ions like magnesium, gadolinium, lanthanum or cisplatin. The aim of this review is to highlight the complex relationship between metal ions and TRP channels.

Flow shear stress enhances intracellular Ca2+ signaling in pulmonary artery smooth muscle cells from patients with pulmonary arterial hypertension

An increase in cytosolic Ca(2+) concentration ([Ca(2+)]cyt) in pulmonary arterial smooth muscle cells (PASMC) is a major trigger for pulmonary vasoconstriction and an important stimulus for pulmonary arterial medial hypertrophy in patients with idiopathic pulmonary arterial hypertension (IPAH). Vascular smooth muscle cells (SMC) sense the blood flow shear stress through interstitial fluid driven by pressure or direct exposure to blood flow in case of endothelial injury. Mechanical stimulus can increase [Ca(2+)]cyt. Here we report that flow shear stress raised [Ca(2+)]cyt in PASMC, while the shear stress-mediated rise in [Ca(2+)]cyt and the protein expression level of TRPM7 and TRPV4 channels were significantly greater in IPAH-PASMC than in normal PASMC. Blockade of TRPM7 by 2-APB or TRPV4 by Ruthenium red inhibited shear stress-induced rise in [Ca(2+)]cyt in normal and IPAH-PASMC, while activation of TRPM7 by bradykinin or TRPV4 by 4αPDD induced greater increase in [Ca(2+)]cyt in IPAH-PASMC than in normal PASMC. The bradykinin-mediated activation of TRPM7 also led to a greater increase in [Mg(2+)]cyt in IPAH-PASMC than in normal PASMC. Knockdown of TRPM7 and TRPV4 by siRNA significantly attenuated the shear stress-mediated [Ca(2+)]cyt increases in normal and IPAH-PASMC. In conclusion, upregulated mechanosensitive channels (e.g., TRPM7, TRPV4, TRPC6) contribute to the enhanced [Ca(2+)]cyt increase induced by shear stress in PASMC from IPAH patients. Blockade of the mechanosensitive cation channels may represent a novel therapeutic approach for relieving elevated [Ca(2+)]cyt in PASMC and thereby inhibiting sustained pulmonary vasoconstriction and pulmonary vascular remodeling in patients with IPAH.

Spatiotemporal expression of TRPM4 in the mouse cochlea

The present study was conducted to elucidate the presence of the transient receptor potential cation channel subfamily M member 4, TRPM4, in the mouse inner ear. TRPM4 immunoreactivity (IR) was found in the cell body of inner hair cells (IHCs) in the organ of Corti in the apical side of marginal cells of the stria vascularis, in the apical portion of the dark cells of the vestibule, and in a subset of the type II neurons in the spiral ganglion. Subsequently, changes in the distribution and expression of TRPM4 in the inner ear during embryonic and postnatal developments were also evaluated. Immunohistochemical localization demonstrated that the emergence of the TRPM4-IR in IHCs occurs shortly before the onset of hearing, whereas that in the marginal cells happens earlier, at the time of birth, coinciding with the onset of endolymph formation. Furthermore, semiquantitative real-time PCR assay showed that expressions of TRPM4 in the organ of Corti and in the stria vascularis increased dramatically at the onset of hearing. Because TRPM4 is a Ca(2+) -activated monovalent-selective cation channel, these findings imply that TRPM4 contributes to potassium ion transport, essential for the signal transduction in IHCs and the formation of endolymph by marginal cells.

Responsiveness of vomeronasal cells to a newt peptide pheromone, sodefrin as monitored by changes of intracellular calcium concentrations

A peptide pheromone of the red-bellied male newt, sodefrin was tested for its ability to increase intracellular concentrations of Ca(2+) ([Ca(2+)]i) in the dissociated vomeronasal (VN) cells of females by means of calcium imaging system. The pheromone elicited a marked elevation of [Ca(2+)]i in a small population of VN cells from sexually developed females. The population of cells exhibiting sodefrin-induced elevation of [Ca(2+)]i increased concentration-dependently. A pheromone of a different species was ineffective in this respect. The VN cells from non-reproductive females or from reproductive males scarcely responded to sodefrin in terms of elevating [Ca(2+)]i. In the cells from hypophysectomized and ovariectomized females, the sodefrin-inducible increase of [Ca(2+)]i never occurred. The cells from the operated newts supplemented with prolactin and estradiol exhibited [Ca(2+)]i responses to sodefrin with a high incidence. Thus, sex- and hormone-dependency as well as species-specificity of the responsiveness of the VN cells to sodefrin was evidenced at the cellular level. Subsequently, possibility of involvement of phospholipase C (PLC)-inositol 1,4,5-trisphosphate (IP3) and/or PLC-diacylglycerol (DAG)-protein kinase C (PKC) pathways in increasing [Ca(2+)]i in VN cells in response to sodefrin was explored using pharmacological approaches. The results indicated that PLC is involved in generating the Ca(2+) signal in all sodefrin-responsive VN cells, whereas IP3 in approximately 50% of the cells and DAG-PKC in the remaining cells. In the latter case, the increase of [Ca(2+)]i was postulated to be induced by the influx of Ca(2+) through the L-type channel. The significance of the finding is discussed.

Mechanisms controlling neurite outgrowth in a pheochromocytoma cell line: the role of TRPC channels

Transient Receptor Potential Canonical (TRPC) channels are implicated in modulating neurite outgrowth. The expression pattern of TRPCs changes significantly during brain development, suggesting that fine-tuning TRPC expression may be important for orchestrating neuritogenesis. To study how alterations in the TRPC expression pattern affect neurite outgrowth, we used nerve growth factor (NGF)-differentiated rat pheochromocytoma 12 (PC12) cells, a model system for neuritogenesis. In PC12 cells, NGF markedly up-regulated TRPC1 and TRPC6 expression, but down-regulated TRPC5 expression while promoting neurite outgrowth. Overexpression of TRPC1 augmented, whereas TRPC5 overexpression decelerated NGF-induced neurite outgrowth. Conversely, shRNA-mediated knockdown of TRPC1 decreased, whereas shRNA-mediated knockdown of TRPC5 increased NGF-induced neurite extension. Endogenous TRPC1 attenuated the anti-neuritogenic effect of overexpressed TRPC5 in part by forming the heteromeric TRPC1-TRPC5 channels. Previous reports suggested that TRPC6 may facilitate neurite outgrowth. However, we found that TRPC6 overexpression slowed down neuritogenesis, whereas dominant negative TRPC6 (DN-TRPC6) facilitated neurite outgrowth in NGF-differentiated PC12 cells. Consistent with these findings, hyperforin, a neurite outgrowth promoting factor, decreased TRPC6 expression in NGF-differentiated PC12 cells. Using pharmacological and molecular biological approaches, we determined that NGF up-regulated TRPC1 and TRPC6 expression via a p75(NTR)-IKK(2)-dependent pathway that did not involve TrkA receptor signaling in PC12 cells. Similarly, NGF up-regulated TRPC1 and TRPC6 via an IKK(2) dependent pathway in primary cultured hippocampal neurons. Thus, our data suggest that a balance of TRPC1, TRPC5, and TRPC6 expression determines neurite extension rate in neural cells, with TRPC6 emerging as an NGF-dependent "molecular damper" maintaining a submaximal velocity of neurite extension.

Pharmacological modulation of diacylglycerol-sensitive TRPC3/6/7 channels

Members of the classic type of transient receptor potential channels (TRPC) represent important molecules involved in hormonal signal transduction. TRPC3/6/7 channels are of particular interest as they are components of phospholipase C driven signalling pathways. Upon receptor-activation, G-protein-mediated stimulation of phospholipase C results in breakdown of phosphatidylinositides leading to increased intracellular diacylglycerol and inositol-trisphosphate levels. Diacylglycerol activates protein kinase C, but more interestingly diacylglycerol directly activates TRPC2/3/6/7 channels. Molecular cloning, expression and characterization of TRP channels enabled reassignment of traditional inhibitors of receptor-dependent calcium entry such as SKF-96365 and 2-APB as blockers of TRPC3/6/7 and several members of non-classic TRP channels. Furthermore, several enzyme inhibitors have also been identified as TRP channel blockers, such as ACA, a phospholipase A₂ inhibitor, and W-7, a calmodulin antagonist. Finally, the naturally occurring secondary plant compound hyperforin has been identified as TRPC6-selective drug, providing an exciting proof of concept that it is possible to generate TRPC-selective channel modulators. The description of Pyr3 as the first TRPC3-selective inhibitor shows that not only nature but also man is able to generate TRP-selective modulators. The review sheds lights on the current knowledge and historical development of pharmacological modulators of TRPC3/6/7. Our analysis indicates that Pyr3 and hyperforin provide promising core structures for the development of new, selective and more potent modulators of TRPC3/6/7 activity.

Fenamates as TRP channel blockers: mefenamic acid selectively blocks TRPM3

BACKGROUND AND PURPOSE

Fenamates are N-phenyl-substituted anthranilic acid derivatives clinically used as non-steroid anti-inflammatory drugs in pain treatment. Reports describing fenamates as tools to interfere with cellular volume regulation attracted our attention based on our interest in the role of the volume-modulated transient receptor potential (TRP) channels TRPM3 and TRPV4.

EXPERIMENTAL APPROACH

Firstly, we measured the blocking potencies and selectivities of fenamates on TRPM3 and TRPV4 as well as TRPC6 and TRPM2 by Ca(2+) imaging in the heterologous HEK293 cell system. Secondly, we further investigated the effects of mefenamic acid on cytosolic Ca(2+) and on the membrane voltage in single HEK293 cells that exogenously express TRPM3. Thirdly, in insulin-secreting INS-1E cells, which endogenously express TRPM3, we validated the effect of mefenamic acid on cytosolic Ca(2+) and insulin secretion.

KEY RESULTS

We identified and characterized mefenamic acid as a selective and potent TRPM3 blocker, whereas other fenamate structures non-selectively blocked TRPM3, TRPV4, TRPC6 and TRPM2.

CONCLUSIONS AND IMPLICATIONS

This study reveals that mefenamic acid selectively inhibits TRPM3-mediated calcium entry. This selectivity was further confirmed using insulin-secreting cells. K(ATP) channel-dependent increases in cytosolic Ca(2+) and insulin secretion were not blocked by mefenamic acid, but the selective stimulation of TRPM3-dependent Ca(2+) entry and insulin secretion induced by pregnenolone sulphate were inhibited. However, the physiological regulator of TRPM3 in insulin-secreting cells remains to be elucidated, as well as the conditions under which the inhibition of TRPM3 can impair pancreatic β-cell function. Our results strongly suggest mefenamic acid is the most selective fenamate to interfere with TRPM3 function.

Stress-induced switch in Numb isoforms enhances Notch-dependent expression of subtype-specific transient receptor potential channel

The Notch signaling pathway plays an essential role in the regulation of cell specification by controlling differentiation, proliferation, and apoptosis. Numb is an intrinsic regulator of the Notch pathway and exists in four alternative splice variants that differ in the length of their phosphotyrosine-binding domain (PTB) and proline-rich region domains. The physiological relevance of the existence of the Numb splice variants and their exact regulation are still poorly understood. We previously reported that Numb switches from isoforms containing the insertion in PTB to isoforms lacking this insertion in neuronal cells subjected to trophic factor withdrawal (TFW). The functional relevance of the TFW-induced switch in Numb isoforms is not known. Here we provide evidence that the TFW-induced switch in Numb isoforms regulates Notch signaling strength and Notch target gene expression. PC12 cells stably overexpressing Numb isoforms lacking the PTB insertion exhibited higher basal Notch activity and Notch-dependent transcription of the transient receptor potential channel 6 (TRPC6) when compared with those overexpressing Numb isoforms with the PTB insertion. The differential regulation of TRPC6 expression is correlated with perturbed calcium signaling and increased neuronal vulnerability to TFW-induced death. Pharmacological inhibition of the Notch pathway or knockdown of TRPC6 function ameliorates the adverse effects caused by the TFW-induced switch in Numb isoforms. Taken together, our results indicate that Notch and Numb interaction may influence the sensitivity of neuronal cells to injurious stimuli by modulating calcium-dependent apoptotic signaling cascades.

Regulation of deoxycholate induction of CXCL8 by the adenomatous polyposis coli gene in colorectal cancer

Elevated deoxycholic acid (DCA), mutations in the adenomatous polyposis coli (APC) gene and chronic inflammation are associated with increased risk of colorectal cancer. APC status was manipulated to determine whether DCA mediates inflammatory molecules in normal or initiated colonic mucosa. DCA increased steady state mRNA and protein levels of CXCL8 in cells which do not express wild-type APC. Steady-state CXCL8 mRNA and protein were suppressed when cells with conditional expression of wild-type APC were exposed to DCA. Immunostaining did not detect CXCL8 in normal human colonic mucosa. CXCL8 was expressed in adenomatous polyps and adenocarcinomas. CXCL8 expression correlated with nuclear beta-catenin localization in epithelial cells of adenomas, but was associated with endothelial cells and neutrophils in the adenocarcinomas. DCA-mediated CXCL8 promoter-reporter activity was elevated in a mutant APC background. Wild-type APC suppressed this effect. Mutation of activator protein-1 (AP-1) or nuclear factor kappa B (NF-kappaB) sites suppressed the activation of the CXCL8 promoter-reporter by DCA. Chromatin immunoprecipitation revealed that AP-1 and NF-kappaB binding to the 5'-promoter of CXCL8 was induced by DCA. The beta-catenin transcription factor was bound to the 5'-promoter of CXCL8 in the absence or presence of DCA. Phenotypic assays determined that DCA-mediated invasion was blocked by antibody-directed against CXCL8 or wild-type APC. CXCL8 exposure led to matrix metalloproteinase-2 production and increased invasion on laminin-coated filters. These data suggest that DCA-mediated CXCL8 occurs in initiated colonic epithelium and neutralizing CXCL8 could reduce the invasive potential of tumors.

Canonical transient receptor potential channels expression is elevated in a porcine model of metabolic syndrome

Plasma epinephrine and heart rate are elevated in metabolic syndrome, suggesting enhanced catecholamine secretion from the adrenal medulla. Canonical transient receptor potential (TRPC) channels are implicated in mediating hormone-induced Ca(2+) influx and catecholamine secretion in adrenomedullary chromaffin cells. We studied the pattern of TRPC expression in the pig adrenal medulla and investigated whether adrenal TRPC expression is altered in prediabetic metabolic syndrome Ossabaw miniature pigs. We used a combination of molecular biological, biochemical, and fluorescence imaging techniques. We determined the sequence of pig TRPC1 and TRPC3-7 channels. We found that the pig adrenal medulla expressed predominantly TRPC1, TRPC5, and TRPC6 transcripts. The expression level of these TRPCs was significantly elevated in the adrenal medulla from pigs with metabolic syndrome. Interestingly, aldosterone, which is endogenously secreted in the adjacent adrenal cortex, increased TRPC1, TRPC5, and TRPC6 expression in adrenal chromaffin cells isolated from metabolic syndrome but not control pigs. Spironolactone, a blocker of mineralocorticoid receptors, inhibited the aldosterone effect. Dexamethasone also increased TRPC5 expression in metabolic syndrome chromaffin cells. The amplitude of hormone-induced divalent cation influx correlated with the level of TRPC expression in adrenal chromaffin cells. Orai1/Stim1 protein expression was not significantly altered in the metabolic syndrome adrenal medulla when compared with the control. We propose that in metabolic syndrome, abnormally elevated adrenal TRPC expression may underlie increased plasma epinephrine and heart rate. The excess of plasma catecholamines and increased heart rate are risk factors for cardiovascular disease. Thus, TRPCs are potential therapeutic targets in the fight against cardiovascular disease.

Heterogeneous distribution of TRPC proteins in the embryonic cortex

The present study was initiated to gain some information about the tissue distribution of transient receptor potential proteins of C-type (TRPC), a family of voltage-independent cation channels, at the beginning of neurogenesis in the telencephalon of embryonic mice. The mRNAs of all known TRPCs (TRPC1-TRPC7) could be found in the cortex at E13. TRPC1, TRPC3 and TRPC5 were the main isoforms, whereas the mRNAs for TRPC2, TRPC4, TRPC6 and TRPC7 were less abundant. The distribution throughout the cortical wall of TRPC1, TRPC3 and TRPC6 was studied by means of immuno-histochemistry. The data collected pointed to a heterogeneous expression of the channels. Three groups were identified. The first one comprises TRPC1, specifically found in the preplate but only in some post-mitotic neurons. It was mainly observed in a subset of cells distinct from the Cajal-Retzius cells. The second group is composed of TRPC3. It was found in non-neuronal cells and also in dividing (5-bromo-2'-deoxyuridine-positive) cells, indicating that TRPC3 is present in precursor cells. The third group contains TRPC6 detected in neuronal and in dividing non-neuronal cells. Double immunostaining experiments showed that TRPC3-positive cells also express TRPC6. Collectively, this report highlights a specific TRPC expression pattern in the immature cortical wall.

Role of TRP channels and NCX in mediating hypoxia-induced [Ca(2+)](i) elevation in PC12 cells

Mammalian cells require a constant O2 supply to produce adequate energy, and sustained hypoxia can kill cells. Mammals therefore have evolved sophisticated mechanisms to allow their cells to adapt to hypoxia. In this study, we investigated the role of TRP channels and the Na+-Ca2+ exchanger (NCX) in mediating hypoxia-induced [Ca2+]i elevation in a model of the O2-sensing rat pheochromocytoma (PC12) cell line by using Ca2+ imaging and molecular biological approaches. Non-selective cation channels, such as TRPC1, 3 and 6, were found to be functionally expressed in PC12 cells. They mediated Ca2+ entry when cells were exposed to acute hypoxia (PO2 of 15 mmHg), in addition to Ca2+ entry via VGCCs. Blockage of TRPCs by 2APB and SKF96365 could significantly reduce hypoxia-mediated [Ca2+]i elevation. Suramin and U73122 attenuated the hypoxia-induced [Ca2+]i elevation, implying the involvement of the G-protein and PLC pathways in the hypoxic response. In addition to TRPCs and VGCCs, NCX also contributed to the hypoxia-induced [Ca2+]i elevation, and blockade of NCX by KBR7943 could significantly decrease the hypoxia-induced [Ca2+]i elevation. Our results suggest that the activation of TRP by hypoxia could lead to NCX reversal; furthermore, membrane depolarization and TRPCs may play a primary role in mediating the hypoxic response in PC12 cells.

Intracellular Ca2+ store depletion induces the formation of macromolecular complexes involving hTRPC1, hTRPC6, the type II IP3 receptor and SERCA3 in human platelets

Endogenously expressed human canonical transient receptor potential 1 (hTRPC1) and human canonical transient receptor potential 6 (hTRPC6) have been shown to play a role in store-operated Ca2+ entry (SOCE) in human platelets, where two mechanisms for SOCE, regulated by the dense tubular system (DTS) or the acidic granules, have been identified. In cells preincubated for 1 min with 100 microM flufenamic acid we show that hTRPC6 is involved in SOCE activated by both mechanisms, as demonstrated by selective depletion of the DTS or the acidic stores, using thapsigargin (TG) (10 nM) or 2,5-di-(tert-butyl)-1,4-hydroquinone (TBHQ) (20 microM), respectively, although it is more relevant after acidic store depletion. Co-immunoprecipitation experiments indicated that depletion of both stores separately results in time-dependent interaction between hTRPC1 and hTRPC6, and also between both hTRPCs and the type II IP3 receptor (IP3RII). The latter was greater after treatment with TG. TBHQ-induced coupling between hTRPC1 and 6 was transient and decreased after 30s of treatment, while that induced by TG increased for at least 3 min. TBHQ induced association between SERCA3, located in the acidic stores, hTRPC1, hTRPC6 and Orai1. TBHQ also evoked coupling between SERCA3 and IP3RII, presumably located in the DTS, thus suggesting interplay between both Ca2+ stores. Similarly, TG induces the interaction of SERCA2b with hTRPC1 and 6 and the IP3RII. The interactions between hTRPC1, hTRPC6, IP3RII and SERCA3 were impaired by disruption of the microtubules, supporting a role for microtubules in Ca2+ homeostasis. In conclusion, the present data demonstrate for the first time that hTRPC1, hTRPC6, IP3RII and SERCA3 are parts of a macromolecular protein complex activated by depletion of the intracellular Ca2+ stores in human platelets.

Chronic hypoxia up-regulates alpha1H T-type channels and low-threshold catecholamine secretion in rat chromaffin cells

alpha(1H) T-type channels recruited by beta(1)-adrenergic stimulation in rat chromaffin cells (RCCs) are coupled to fast exocytosis with the same Ca(2+) dependence of high-threshold Ca(2+) channels. Here we show that RCCs exposed to chronic hypoxia (CH) for 12-18 h in 3% O(2) express comparable densities of functional T-type channels that depolarize the resting cells and contribute to low-voltage exocytosis. Following chronic hypoxia, most RCCs exhibited T-type Ca(2+) channels already available at -50 mV with the same gating, pharmacological and molecular features as the alpha(1H) isoform. Chronic hypoxia had no effects on cell size and high-threshold Ca(2+) current density and was mimicked by overnight incubation with the iron-chelating agent desferrioxamine (DFX), suggesting the involvement of hypoxia-inducible factors (HIFs). T-type channel recruitment occurred independently of PKA activation and the presence of extracellular Ca(2+). Hypoxia-recruited T-type channels were partially open at rest (T-type 'window-current') and contributed to raising the resting potential to more positive values. Their block by 50 microm Ni(2+) caused a 5-8 mV hyperpolarization. The secretory response associated with T-type channels could be detected following mild cell depolarizations, either by capacitance increases induced by step depolarizations or by amperometric current spikes induced by increased [KCl]. In the latter case, exocytotic bursts could be evoked even with 2-4 mm KCl and spike frequency was drastically reduced by 50 microm Ni(2+). Chronic hypoxia did not alter the shape of spikes, suggesting that hypoxia-recruited T-type channels increase the number of secreted vesicles at low voltages, without altering the mechanism of catecholamine release and the quantal content of released molecules.

Induction of calcium influx through TRPC5 channels by cross-linking of GM1 ganglioside associated with alpha5beta1 integrin initiates neurite outgrowth

Previous studies demonstrated that cross-linking of GM1 ganglioside with multivalent ligands, such as B subunit of cholera toxin (CtxB), induced Ca2+ influx through an unidentified, voltage-independent channel in several cell types. Application of CtxB to undifferentiated NG108-15 cells resulted in outgrowth of axon-like neurites in a Ca2+ influx-dependent manner. In this study, we demonstrate that CtxB-induced Ca2+ influx is mediated by TRPC5 channels, naturally expressed in these cells and primary neurons. Both Ca2+ influx and neurite induction were blocked by TRPC5 small interfering RNA (siRNA). Pretreatment of NG108-15 cells with neuraminidase increased cell-surface GM1 and greatly enhanced the signal. GM1 was not directly associated with TRPC5 but rather with alpha5beta1 integrin, which opened the channel through a signaling sequence after cross-linking of the GM1/integrin complex. This cascade included autophosphorylation of focal adhesion kinase and subsequent activation of phospholipase Cgamma (PLCgamma) and phosphoinositide-3 kinase [PI(3)K]. Pharmacological blockers that inhibited tyrosine kinase, PLC, and PI(3)K suppressed both CtxB-induced Ca2+ influx and neurite outgrowth. These were also suppressed by SK&F96365, a nonspecific transient receptor potential channel blocker. Confocal immunocytochemistry revealed that GM1 cross-linking induced colocalization of GM1 with these signaling elements in sprouting regions of plasma membrane. In primary cerebellar granular neurons (CGNs), TRPC5 was detected at 2 d in vitro (2 DIV), a stage corresponding to CtxB-stimulated Ca2+ influx. Neurite outgrowth in CGNs, determined at 3 DIV, was accelerated by CtxB and suppressed by TRPC5 siRNA and the above blockers. The crucial role of GM1 was indicated with CGNs from ganglio-series null mice, in which growth of axons was significantly retarded.

Transient Receptor Potential Channels and Intracellular Signaling

The transient receptor potential (TRP) family of ion channels is composed of more than 50 functionally versatile cation-permeant ion channels expressed in most mammalian cell types. Considerable research has been brought to bear on the members of this family, especially with regard to their possible role as store-operated calcium channels, although studies have provided evidence that TRP channels exhibit a number of regulatory and functional aspects. Endogenous and transiently expressed TRP channels can be activated by different mechanisms grouped into four main categories: receptor-operated activation, store depletion-mediated activation, ligand-induced activation, and direct activation. This article reviews the biochemical characteristics of the different members of the TRP family and summarizes their involvement in a number of physiological events ranging from sensory transduction to development, which might help in understanding the relationship between TRP channel dysfunction and the development of several diseases.

Regulation of OX1 orexin/hypocretin receptor-coupling to phospholipase C by Ca2+ influx

BACKGROUND AND PURPOSE

Orexin (OX) receptors induce Ca2+ elevations via both receptor-operated Ca2+ channels (ROCs) and the "conventional" phospholipase C (PLC)-Ca2+ release-store-operated Ca2+ channel (SOC) pathways. In this study we assessed the ability of these different Ca2+ influx pathways to amplify OX1 receptor signalling to PLC in response to stimulation with the physiological ligand orexin-A.

EXPERIMENTAL APPROACH

PLC activity was assessed in CHO cells stably expressing human OX1 receptors.

KEY RESULTS

Inhibition of total Ca2+ influx by reduction of the extracellular [Ca2+] to 1 microM effectively inhibited the receptor-stimulated PLC activity at low orexin-A concentrations (by 93% at 1 nM), and this effect was gradually reduced by higher orexin-A concentrations. A similar but weaker inhibitory effect (84% at 1 nM) was obtained on depolarization to approximately 0 mV, which disrupts most of the driving force for Ca2+ entry. The inhibitor of the OX1 receptor-activated ROCs, tetraethylammonium chloride (TEA), was somewhat less effective than the reduction in extracellular [Ca2+] at inhibiting PLC activation, probably because it only partially blocks ROCs. The partial inhibitor of both ROCs and SOCs, Mg2+, and the SOC inhibitors, dextromethorphan, SKF-96365 (1-[beta-(3-(4-methoxyphenyl)propoxy)-4-methoxyphenethyl]-1H-imidazole HCL) and 2-APB (2-aminoethoxydiphenyl borate), inhibited PLC activity at low concentrations of orexin-A, but were not as effective as TEA.

CONCLUSIONS AND IMPLICATIONS

Both ROCs and SOCs markedly amplify the OX(1) receptor-induced PLC response, but ROCs are more central for this response. These data indicate the crucial role of ROCs in orexin receptor signalling.

The orexin OX1 receptor regulates Ca2+ entry via diacylglycerol-activated channels in differentiated neuroblastoma cells

We studied the cellular response to orexin type 1 receptor (OX1R) stimulation in differentiated IMR-32 neuroblastoma cells. In vitro differentiation of IMR-32 cells with 5-bromo-2'-deoxyuridine leads to a neuronal phenotype with long neurite extensions and an upregulation of mainly N-type voltage-gated calcium channels. Transduction of differentiated IMR-32 cells with baculovirus harboring an OX1R-green fluorescent protein cDNA fusion construct resulted in appearance of fluorescence that was confined mainly to the plasma membrane in the cell body and to neurites. Application of orexin-A to fluorescent cells led to an increase in intracellular free Ca2+ concentration, [Ca2+]i. At low nanomolar concentrations of orexin-A, the response was reversibly attenuated by removal of extracellular Ca2+, by application of a high concentration (10 mM) of Mg2+, and by the pharmacological channel blocker dextromethorphan. A diacylglycerol, dioctanoylglycerol, but not thapsigargin or depolarization with potassium, mimicked the OX1R response with regard to Mg2+ sensitivity. A reverse transcription-PCR screening identified mRNAs for all transient receptor potential canonical (TRPC) channels, including TRPC3, TRPC6, and TRPC7, which are known to be activated by diacylglycerol. Expression of a dominant-negative TRPC6 channel subunit blunted the responses to both dioctanoylglycerol and OX1R stimulation. The results suggest that the OX1R activates a Ca2+ entry pathway that involves diacylglycerol-activated TRPC channels in neuronal cells.

A "sample-and-hold" pulse-counting integrator as a mechanism for graded memory underlying sensorimotor adaptation

The mechanisms behind the induction of cellular correlates of memory by sensory input and their contribution to meaningful behavioral changes are largely unknown. We previously reported a graded memory in the form of sensorimotor adaptation in the electromotor output of electric fish. Here we show that the mechanism for this adaptation is a synaptically induced long-lasting shift in intrinsic neuronal excitability. This mechanism rapidly integrates hundreds of spikes in a second, or gradually integrates the same number of spikes delivered over tens of minutes. Thus, this mechanism appears immune to frequency-dependent fluctuations in input and operates as a simple pulse counter over a wide range of time scales, enabling it to transduce graded sensory information into a graded memory and a corresponding change in the behavioral output. This adaptation is based on an NMDA receptor-mediated change in intrinsic excitability of the postsynaptic neurons involving the Ca2+-dependent activation of TRP channels.

Compartmentalization of calcium entry pathways in mouse rods

Photoreceptor metabolism, gene expression and synaptic transmission take place in a highly polarized structure consisting of the ellipsoid, subellipsoid, cell body and synaptic terminal regions. Although calcium, a key second messenger, regulates cellular functions throughout the photoreceptor, the molecular mechanisms underlying local region-specific action of Ca2+ in photoreceptors are poorly understood. I have investigated the compartmentalization of voltage-dependent and independent Ca2+ channels in mouse photoreceptors. Transient receptor potential channels isoform 6 (TRPC6), a putative store-operated Ca2+ channel, was selectively localized to the cell body of rods. By contrast, voltage-operated Ca2+ channels were expressed in the synaptic terminal and in the ellipsoid/subellipsoid regions. Likewise, Ca2+ store transporters and channels were strongly associated with the subellipsoid region. A moderate TRPC6 signal was observed in cell bodies of bipolar, amacrine and ganglion cells, but was absent from both plexiform layers. These results suggest that Ca2+ entry mechanisms in mammalian photoreceptors and bipolar cells are highly compartmentalized, consistent with local, region-specific activation of Ca2+-dependent processes.

Human TRPC6 expressed in HEK 293 cells forms non-selective cation channels with limited Ca2+ permeability

TRPC6 is thought to be a Ca(2+)-permeable cation channel activated following stimulation of G-protein-coupled membrane receptors linked to phospholipase C (PLC). TRPC6 current is also activated by exogenous application of 1-oleoyl-acetyl-sn-glycerol (OAG) or by inhibiting 1,2-diacylglycerol (DAG) lipase activity using RHC80267. In the present study, both OAG and RHC80267 increased whole-cell TRPC6 current in cells from a human embryonic kidney cell line (HEK 293) stably expressing TRPC6, but neither compound increased cytosolic free Ca(2+) concentration ([Ca(2+)](i)) when the cells were bathed in high-K(+) buffer to hold the membrane potential near 0 mV. These results suggested that TRPC6 channels have limited Ca(2+) permeability relative to monovalent cation permeability and/or that Ca(2+) influx via TRPC6 is greatly attenuated by depolarization. To evaluate Ca(2+) permeability, TRPC6 currents were examined in extracellular buffer in which Ca(2+) was varied from 0.02 to 20 mm. The results were consistent with a pore-permeation model in which Ca(2+) acts primarily as a blocking ion and contributes only a small percentage ( approximately 4%) to whole-cell currents in the presence of extracellular Na(+). Measurement of single-cell fura-2 fluorescence during perforated-patch recording of TRPC6 currents showed that OAG increased [Ca(2+)](i) 50-100 nm when the membrane potential was clamped at between -50 and -80 mV, but had little or no effect if the membrane potential was left uncontrolled. These results suggest that in cells exhibiting a high input resistance, the primary effect of activating TRPC6 will be membrane depolarization. However, in cells able to maintain a hyperpolarized potential (e.g. cells with a large inwardly rectifying or Ca(2+)-activated K(+) current), activation of TRPC6 will lead to a sustained increase in [Ca(2+)](i). Thus, the contribution of TRPC6 current to both the kinetics and magnitude of the Ca(2+) response will be cell specific and dependent upon the complement of other channel types.

Regulation of TRPM2 channels in neutrophil granulocytes by ADP-ribose: a promising pharmacological target

TRPM2 channels play an important role in the activation process of neutrophil granulocytes. One mechanism of TRPM2 channel gating is the binding of intracellular ADP ribose (ADPR) to the Nudix box domain in the C-terminal tail of TRPM2. Intracellular Ca(2+), although not an activator of TRPM2 by its own, significantly enhances TRPM2 gating by ADPR. Stimulation of neutrophil granulocytes with the chemoattractant peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP) induces release of Ca(2+) ions from intracellular stores which in cooperation with endogenous ADPR levels enable Ca(2+) influx through TRPM2. Stimulation of the ectoenzyme CD38, a membrane-associated glycohydrolase with ADPR as main product, and uptake of ADPR into the cell may contribute to the effects of fMLP. Inhibition of ADPR production, of uptake and of binding to TRPM2 are all potential pharmacological principles by which a modulation of neutrophil function may become possible in future.

Physiological mechanisms of TRPC activation

TRPC (canonical transient receptor potential) channels are vertebrate homologs of the Drosophila photoreceptor channel, TRP. Considerable research has been brought to bear on the seven members of this family, especially with regard to their possible role in calcium entry. Unfortunately, the current literature presents a confusing picture, with different laboratories producing widely differing results and interpretations. It appears that ectopically expressed TRPC channels can be activated by phospholipase C products (generally, diacylglycerols), by stimulation of trafficking to the plasma membrane, or by depletion of intracellular Ca(2+) stores. Here, I discuss the possibility that these diverse experimental findings arise because TRPC channels can, under both experimental as well as physiological conditions, be activated in three distinct ways, possibly depending on their subunit composition and/or signaling complex environment. The TRPCs may be unique among ion-channel subunit families in being able to participate in the assembly and function of multiple types of physiologically important ion channels.

Hyperforin activates nonselective cation channels (NSCCs)

A large body of evidence supports the preclinical antidepressant profile of hyperforin including inhibition of the synaptosomal uptake of several neurotransmitters by hyperforin and studies in behavioural models. In contrast to other antidepressants, hyperforin does not directly inhibit neurotransmitter transporters, but instead uptake inhibition seems to be the consequence of an elevated intracellular sodium concentration ([Na+]i). The mechanism of hyperforin-induced elevation of [Na+]i was investigated using two different cell types: human platelets and rat pheochromocytoma cells (PC12 cells). In both cell systems, hyperforin increased both [Na+]i and free intracellular Ca2+ concentration ([Ca2+]i). One pathway for Na+ and Ca2+ entry is mediated by nonselective cation channels (NSCCs), which can be blocked by SK&F 96365 and LOE 908. LOE 908 is a blocker of both NSCC1 and NSCC2 subclasses, while SK&F 96365 blocks NSCC2 only. Both SK&F 96365 and LOE 908 completely inhibited the hyperforin-induced influx of Na+ and Ca2+ into platelets and PC12 cells. This indicates that hyperforin is mainly active upon NSCC2. The effect of hyperforin is inhibited by La3+ and Gd3+, indicating that there is a potential homology with canonical transient receptor potential protein channels (TRPC channels). Moreover, La3+ and Gd3+ attenuate the effect of hyperforin on serotonin uptake in human platelets. Additionally, hyperforin induces barium influx in PC12 cells and this influx can be inhibited by SK&F 96365, LOE 908, Gd3+ and La3+. In summary, these findings suggest that hyperforin represents a new principle for preclinical antidepressant activity, modulating brain neurotransmission by inhibition of neurotransmitter uptake via activation of NSCCs.British Journal of Pharmacology (2005) 145, 75-83. doi:10.1038/sj.bjp.0706155.

Mechanistic and functional changes in Ca2+ entry after retinoic acid-induced differentiation of neuroblastoma cells

We have investigated effects of neuronal differentiation on hormone-induced Ca2+ entry. Fura-2 fluorescence measurements of undifferentiated SH-SY5Y neuroblastoma cells, stimulated with methacholine, revealed the presence of voltage-operated Ca2+-permeable, Mn2+-impermeable entry pathways, and at least two voltage-independent Ca2+- and Mn2+-permeable entry pathways, all of which apparently contribute to both peak and plateau phases of the Ca2+ signal. Similar experiments using 9-cis retinoic acid-differentiated cells, however, revealed voltage-operated Ca2+-permeable, Mn2+-impermeable channels, and, more significantly, the absence or down-regulation of the most predominant of the voltage-independent entry pathways. This down-regulated pathway is probably due to CCE (capacitative Ca2+ entry), since thapsigargin also stimulated Ca2+ and Mn2+ entry in undifferentiated but not differentiated cells. The Ca2+ entry components remaining in methacholine-stimulated differentiated cells contributed to only the plateau phase of the Ca2+ signal. We conclude that differentiation of SH-SY5Y cells results in a mechanistic and functional change in hormone-stimulated Ca2+ entry. In undifferentiated cells, voltage-operated Ca2+ channels, CCE and NCCE (non-CCE) pathways are present. Of the voltage-independent pathways, the predominant one appears to be CCE. These pathways contribute to both peak and plateau phases of the Ca2+ signal. In differentiated cells, CCE is either absent or down-regulated, whereas voltage-operated entry and NCCE remain active and contribute to only the plateau phase of the Ca2+ signal.

Role of phospholipase D and diacylglycerol in activating constitutive TRPC-like cation channels in rabbit ear artery myocytes

Previously we have described a constitutively active Ca2+-permeable non-selective cation channel in freshly dispersed rabbit ear artery myocytes that has similar properties to canonical transient receptor potential (TRPC) channel proteins. In the present study we have investigated the transduction pathways responsible for stimulating constitutive channel activity in these myocytes. Application of the pharmacological inhibitors of phosphatidylcholine-phospholipase D (PC-PLD), butan-1-ol and C2 ceramide, produced marked inhibition of constitutive channel activity in cell-attached patches and also butan-1-ol produced pronounced suppression of resting membrane conductance measured with whole-cell recording whereas the inactive isomer butan-2-ol had no effect on constitutive whole-cell or channel activity. In addition butan-1-ol had no effect on channel activity evoked by the diacylglycerol (DAG) analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG). Inhibitors of PC-phospholipase C (PC-PLC) and phospholipase A2 (PLA2) had no effect on constitutive channel activity. Application of a purified PC-PLD enzyme and its metabolite phosphatidic acid to inside-out patches markedly increased channel activity. The phosphatidic acid phosphohydrolase (PAP) inhibitor dl-propranolol also inhibited constitutive and phosphatidic acid-induced increases in channel activity but had no effect on OAG-evoked responses. The DAG lipase and DAG kinase inhibitors, RHC80267 and R59949 respectively, which inhibit DAG metabolism, produced transient increases in channel activity which were mimicked by relatively high concentrations (40 microm) of OAG. The protein kinase C (PKC) inhibitor chelerythrine did not prevent channel activation by OAG but blocked the secondary inhibitory response of OAG. It is proposed that endogenous DAG is involved in the activation of channel activity and that its effects on channel activity are concentration-dependent with higher concentrations of DAG also inhibiting channel activity through activation of PKC. This study indicates that constitutive cation channel activity in ear artery myocytes is mediated by DAG which is generated by PC-PLD via phosphatidic acid which represents a novel activation pathway of cation channels in vascular myocytes.

Transient receptor potential and other ion channels as pharmaceutical targets in airway smooth muscle cells

Regardless of the triggering stimulus in asthma, contraction of the airway smooth muscle (ASM) is considered to be an important pathway leading to the manifestation of asthmatic symptoms. Therefore, the various ion channels that modulate ASM contraction and relaxation are particularly attractive targets for therapy. Although voltage-operated Ca2+ channels (VOCC) are the most extensively characterised Ca(2+)-permeable channels in ASM cells and are obvious pharmacological targets, blockers of VOCC have not been successful in alleviating ASM contraction in asthma. Similarly, although the Cl- and K+ channels also modulate ASM contraction and relaxation by regulating plasma membrane potential, pharmacological interventions directed against these channels have failed to abrogate ASM contraction in asthma. A large body of evidence suggests that store-operated Ca2+ channels (SOCC) and Ca(2+)-permeable second messenger-activated non-selective cation channels (NSCC) predominantly mediate ASM contraction. However, development of pharmacological interventions involving these channels has been hampered by the paucity of information regarding their molecular identity. Members of the mammalian transient receptor potential (TRP) protein family, which form voltage-independent channels with variable Ca2+ selectivity that are activated by store depletion and/or by intracellular messengers, are potential molecular candidates for SOCC and NSCC in ASM cells. While the function of TRP channels in ASM cells remains to be elucidated and there are, at present, essentially no good TRP channel antagonists, this group of proteins is a potentially valuable pharmaceutical target for the treatment of asthma.

Calmidazolium and arachidonate activate a calcium entry pathway that is distinct from store-operated calcium influx in HeLa cells

Agonists that deplete intracellular Ca2+ stores also activate Ca2+ entry, although the mechanism by which store release and Ca2+ influx are linked is unclear. A potential mechanism involves 'store-operated channels' that respond to depletion of the intracellular Ca2+ pool. Although SOCE (store-operated Ca2+ entry) has been considered to be the principal route for Ca2+ entry during hormonal stimulation of non-electrically excitable cells, recent evidence has suggested that alternative pathways activated by metabolites such as arachidonic acid are responsible for physiological Ca2+ influx. It is not clear whether such messenger-activated pathways exist in all cells, whether they are truly distinct from SOCE and which metabolites are involved. In the present study, we demonstrate that HeLa cells express two pharmacologically and mechanistically distinct Ca2+ entry pathways. One is the ubiquitous SOCE route and the other is an arachidonate-sensitive non-SOCE. We show that both these Ca2+ entry pathways can provide long-lasting Ca2+ elevations, but that the channels are not the same, based on their differential sensitivity to 2-aminoethoxydiphenyl borate, LOE-908 [(R,S)-(3,4-dihydro-6,7-dimethoxy-isochinolin-1-yl)-2-phenyl-N,N-di[2-(2,3,4-trimethoxyphenyl)ethyl]acetamid mesylate] and gadolinium. In addition, non-SOCE and not SOCE was permeable to strontium. Furthermore, unlike SOCE, the non-SOCE pathway did not require store depletion and was not sensitive to displacement of the endoplasmic reticulum from the plasma membrane using jasplakinolide or ionomycin pretreatment. These pathways did not conduct Ca2+ simultaneously due to the dominant effect of arachidonate, which rapidly curtails SOCE and promotes Ca2+ influx via non-SOCE. Although non-SOCE could be activated by exogenous application of arachidonate, the most robust method for stimulation of this pathway was application of the widely used calmodulin antagonist calmidazolium, due to its ability to activate phospholipase A2.

Transient receptor potential-like channels mediate metabotropic glutamate receptor EPSCs in rat dopamine neurones

Transient receptor potential (TRP) channels form cationic channels activated by diverse factors including mechanical stimuli, changes in osmolarity, pH and temperature, as well as the exogenous irritant, capsaicin. Metabotropic glutamate receptors have also recently been linked to TRP channel activation in neurones of the substantia nigra, hippocampus and cerebellum, suggesting a novel role for such channels in synaptic communication via endogenous neurotransmitters. We tested this for dopamine neurones in rat brain slices by characterizing the current-voltage relationship and pharmacology of EPSCs mediated by group I metabotropic glutamate receptor subtype 1 (mGluR1). Slow inward currents (273 +/- 35 pA peak amplitude, 381 +/- 25 ms latency, holding potential (V(h)) =-73 mV) representing evoked mGluR1 EPSCs were isolated in the presence of antagonists of AMPA, NMDA, GABA(A), GABA(B), muscarinic and glycine receptors. CPCCOEt (100 microM), an mGluR1 antagonist, blocked the residual EPSC in all recordings. mGluR1-activated EPSCs reversed polarity near -10 mV, consistent with the involvement of a cationic channel. Extracellular application of the non-selective TRP channel blockers SKF 96365, flufenamic acid and ruthenium red caused reversible inhibition of mGluR1-activated EPSCs. These characteristics parallel those of mGluR1 activation with an agonist and indicate the involvement of a TRP-like channel in mGluR1-mediated EPSCs.

Involvement of transient receptor potential-like channels in responses to mGluR-I activation in midbrain dopamine neurons

We investigated the involvement of store-operated channels (SOCs) and transient receptor potential (TRP) channels in the response to activation of the group I metabotropic glutamate receptor subtype 1 (mGluR1) with the agonist (S)-3,5-dihydroxyphenylglycine (DHPG, puff application) in dopamine neurons in rat brain slices. The mGluR1-induced conductance reversed polarity close to 0 mV and at more positive potentials when extracellular potassium concentrations were increased, indicating the involvement of a cationic channel. DHPG currents but not intracellular calcium responses were reduced by low extracellular sodium concentrations but were not affected by sodium channel blockers, tetrodotoxin and saxitoxin or by inhibition of the h-current with cesium. Abolition of calcium responses with intracellular BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; 10 mm) did not affect current responses, indicating they were not calcium activated. Extracellular application of non-selective SOCs and TRP channel blockers 2-aminoethoxydiphenylborane (2-APB), SKF96365, ruthenium red and flufenamic acid (but not gadolinium) reduced DHPG current and calcium responses. Intracellular application of ruthenium red and 2-APB did not affect DHPG currents, indicating that IP3 and ryanodine receptors did not mediate their actions. Single-cell PCR revealed the presence of TRPC1 and 5 mRNA in most dopamine neurons and subtypes 3, 4 and 6 in some. Store depletion evoked calcium entry indicative of SOCs, providing the first functional observation of such channels in native central neurons. Store depletion with either cyclopiazonic acid or ryanodine abolished calcium but not current responses to DHPG. The electrophysiological and pharmacological properties of the mGluR1-induced inward current are consistent with the involvement of TRP channels whereas calcium responses are dependent on the function of SOCs in voltage clamp recordings.

Site-directed mutagenesis of the active site of diacylglycerol kinase alpha: calcium and phosphatidylserine stimulate enzyme activity via distinct mechanisms

Diacylglycerol kinases (DAGKs) catalyse ATP-dependent phosphorylation of sn-1,2-diacylglycerol that arises during stimulated phosphatidylinositol turnover. DAGKa is activated in vitro by Ca2+ and by acidic phospholipids. The regulatory region of DAGKa includes an N-terminal RVH motif and EF hands that mediate Ca2+-dependent activation. DAGKa also contains tandem C1 protein kinase C homology domains. We utilized yeast, Saccharomyces cerevisiae, which lacks an endogenous DAGK, to express DAGKa and to determine the enzymic activities of different mutant forms of pig DAGKa in vitro. Six aspartate residues conserved in all DAGKs were individually examined by site-directed mutagenesis. Five of these aspartate residues reside in conserved blocks that correspond to sequences in the catalytic site of phosphofructokinases. Mutation of D434 (Asp434) or D650 abolished all DAGKa activity, whereas substitution of one among D465, D497, D529 and D697 decreased the activity to 6% or less of that for wild-type DAGKa. Roles of homologous residues in phosphofructokinases suggested that the N-terminal half of the DAGK catalytic domain binds Mg-ATP and the C-terminal half binds diacylglycerol. A DAGKa mutant with its entire regulatory region deleted showed a much decreased activity that was not activated by Ca2+, but still exhibited PS (phosphatidylserine)-dependent activation. Moreover, mutations of aspartate residues at the catalytic domain had differential effects on activation by Ca2+ and PS. These results indicate that Ca2+ and PS stimulate DAGKa via distinct mechanisms.

Regulation of TRP channels via lipid second messengers

In Drosophila photoreceptors, the light-sensitive current is mediated downstream of phospholipase C by TRP (transient receptor potential) channels. Recent evidence suggests that Drosophila TRP channels are activated by diacylglycerol (DAG) or its metabolites (polyunsaturated fatty acids), possibly in combination with the reduction in phosphatidyl inositol 4,5 bisphosphate (PIP2). Consistent with this view, diacylglycerol kinase is identified as a key enzyme required for response termination. Signaling is critically dependent upon efficient PIP2 synthesis; mutants of this pathway in combination with genetically targeted PIP2 reporters provide unique insights into the kinetics and regulation of PIP2 turnover. Recent evidence indicates that a growing number of mammalian TRP homologues are also regulated by lipid messengers, including DAG, arachidonic acid, and PIP2.

Alpha1-adrenergic receptors activate Ca(2+)-permeable cationic channels in prostate cancer epithelial cells

The prostate gland is a rich source of alpha1-adrenergic receptors (alpha1-ARs). alpha1-AR antagonists are commonly used in the treatment of benign prostatic hyperplasia symptoms, due to their action on smooth muscle cells. However, virtually nothing is known about the role of alpha1-ARs in epithelial cells. Here, by using two human prostate cancer epithelial (hPCE) cell models - primary cells from resection specimens (primary hPCE cells) and an LNCaP (lymph node carcinoma of the prostate) cell line - we identify an alpha1A subtype of adrenergic receptor (alpha1A-AR) and show its functional coupling to plasmalemmal cationic channels via direct diacylglycerol (DAG) gating. In both cell types, agonist-mediated stimulation of alpha1A-ARs and DAG analogues activated similar cationic membrane currents and Ca(2+) influx. These currents were sensitive to the alpha1A-AR antagonists, prazosin and WB4101, and to transient receptor potential (TRP) channel blockers, 2-aminophenyl borate and SK&F 96365. Chronic activation of alpha1A-ARs enhanced LNCaP cell proliferation, which could be antagonized by alpha1A-AR and TRP inhibitors. Collectively, our results suggest that alpha1-ARs play a role in promoting hPCE cell proliferation via TRP channels.

Receptor-coupled, DAG-gated Ca2+-permeable cationic channels in LNCaP human prostate cancer epithelial cells

Although the prostate gland is a rich source of alpha1-adreno- (alpha1-AR) and m1-cholino receptors (m1-AChR), the membrane processes associated with their activation in glandular epithelial cells is poorly understood. We used the whole-cell patch-clamp technique to show that the agonists of the respective receptors, phenylephrine (PHE) and carbachol (CCh), activate cationic membrane currents in lymph node carcinoma of the prostate (LNCaP) human prostate cancer epithelial cells, which are not dependent on the filling status of intracellular IP3-sensitive Ca2+ stores, but directly gated by diacylglycerol (DAG), as evidenced by the ability of its membrane permeable analogue, OAG, to mimic the effects of the agonists. The underlying cationic channels are characterized by the weak field-strength Eisenman IV permeability sequence for monovalent cations (PK(25) > PCs(4.6) > PLi(1.4) > PNa(1.0)), and the following permeability sequence for divalent cations: PCa(1.0) > PMg(0.74) > PBa(0.6) > PSr(0.36) > PMn(0.3). They are 4.3 times more permeable to Ca2+ than Na+ and more sensitive to the inhibitor 2-APB than SK&F 96365. RT-PCR analysis shows that DAG-gated members of the transient receptor potential (TRP) channel family, including TRPC1 and TRPC3, are present in LNCaP cells. We conclude that, in prostate cancer epithelial cells, alpha1-ARs and m1-AChRs are functionally coupled to Ca2+-permeable DAG-gated cationic channels, for which TRPC1 and TRPC3 are the most likely candidates.

Mechanisms in histamine-mediated secretion from adrenal chromaffin cells

The great majority of the sustained secretory response of adrenal chromaffin cells to histamine is due to extracellular Ca(2+) influx through voltage-operated Ca(2+) channels (VOCCs). This is likely to be true also for other G protein-coupled receptor (GPCR) agonists that evoke catecholamine secretion from these cells. However, the mechanism by which these GPCRs activate VOCCs is not yet clear. A substantial amount of data have established that histamine acts on H(1) receptors to activate phospholipase C via a Pertussis toxin-resistant G protein, causing the production of inositol 1,4,5-trisphosphate and the mobilisation of store Ca(2+); however, the molecular events that lead to the activation of the VOCCs remain undefined. This review will summarise the known actions of histamine on cellular signalling pathways in adrenal chromaffin cells and relate them to the activation of extracellular Ca(2+) influx through voltage-operated channels, which evokes catecholamine secretion. These actions provide insight into how other GPCRs might activate Ca(2+) influx in many excitable and non-excitable cells.

Evidence for the expression of transient receptor potential proteins in guinea pig airway smooth muscle cells

OBJECTIVE

The present study investigates the expression of transient receptor potential (TRPC) proteins in airway smooth muscle (ASM) cells in order to determine whether these proteins may be candidate molecular counterparts of plasma membrane Ca2+-permeable channels involved in the contraction of ASM.

METHODS

Expression of TRPC mRNA was detected using specific primers and RT-PCR. Expression of the TRPC1, TRPC3 and TRPC6 proteins was detected using antibodies in immunoprecipitation and Western blot.

RESULTS

Guinea pig ASM cells exhibited thapsigargin- and acetylcholine-initiated Ca2+ inflow but none by 1-oleoyl-2-acetyl-sn-glycerol. mRNA encoding each of the TRPC1 to TRPC6 proteins was detected in ASM cells. mRNA encoding TRPC1, TRPC3, TRPC4 and TRPC6 was detected in ASM cells at a concentration approximately equivalent to that in guinea pig brain. mRNA encoding TRPC2 and TRPC5 was more abundant in ASM cells than in brain. The TRPC1 protein, but not the TRPC3 or TRPC6 proteins, was detected in extracts of ASM cells, while all three proteins were detected in brain.

CONCLUSION

The results provide evidence for a low level of expression of the TRPC1 to TRPC6 proteins in ASM cells. These proteins may function as store-operated Ca2+ and/or second messenger-activated non-selective cation channels in ASM cells.

Muscarinic regulation of pacemaker frequency in murine gastric interstitial cells of Cajal

Peristaltic contractions in the stomach are regulated by the spread of electrical slow waves from the corpus to the pylorus. Gastric slow waves are generated and propagated by the interstitial cells of Cajal (ICC). All regions distal to the dominant pacemaker area in the corpus are capable of generating slow waves, but orderly gastric peristalsis depends upon a frequency gradient in which the corpus pacemaker frequency exceeds the antral frequency. Cholinergic, muscarinic stimulation enhances pacemaker frequency. We investigated this phenomenon using intact murine gastric muscles and cultured ICC. Acetylcholine (ACh) increased the frequency of slow waves in antrum and corpus muscles. The increase was significantly greater in the antrum. ACh and carbachol (CCh) increased the pacemaker currents in cultured ICC. At high doses of CCh, transient pacemaker currents fused into sustained inward currents that persisted for the duration of stimulation. The effects of CCh were blocked by low doses of the M(3) receptor antagonist 1-dimethyl-4-diphenylacetoxypiperidinium. Frequency enhancement by CCh was not affected by forskolin, but the phospholipase C inhibitor U-73122 inhibited both the increase in frequency and the development of tonic inward currents. 2-Aminoethyldiphenyl borate also blocked the chronotropic responses to CCh. Inhibitors of protein kinase C did not block responses to CCh. These studies show that mice are an excellent model for studying mechanisms that regulate gastric slow-wave frequency. CCh, apparently via production of inositol 1,4,5-trisphosphate, accelerates the frequency of pacemaker activity. High concentrations of CCh may block the entrainment of pacemaker currents, resulting in a tonic inward current.

Diacylglycerol activates the influx of extracellular cations in T-lymphocytes independently of intracellular calcium-store depletion and possibly involving endogenous TRP6 gene products

In Jurkat and human peripheral blood T-lymphocytes, 1-oleoyl-2-acetyl-sn-glycerol (OAG), a membrane-permeant analogue of diacylglycerol, activated the influx of Ca(2+), Ba(2+) and Sr(2+). OAG also caused plasma-membrane depolarization in Ca(2+)-free media that was recovered by the addition of bivalent cation, indicating the activation of Na(+) influx. OAG-induced cation influx was (i) mimicked by the natural dacylglycerol 1-stearoyl-2-arachidonyl-sn-glycerol, (ii) not blocked by inhibiting protein kinase C or in the absence of phospholipase C activity and (iii) blocked by La(3+) and Gd(3+). Differently from OAG, both thapsigargin and phytohaemagglutinin activated a potent influx of Ca(2+), but little influx of Ba(2+) and Sr(2+). Moreover, the influx of Ca(2+) activated by thapsigargin and that activated by OAG were additive. Furthermore, several drugs (i.e. econazole, SKF96365, carbonyl cyanide p-trifluoromethoxyphenylhydrazone, 2-aminoethoxy diphenylborate and calyculin-A), while inhibiting the influx of Ca(2+) induced by both thapsigargin and phytohaemagglutinin, did not affect OAG-stimulated cation influx. Transient receptor potential (TRP) 3 and TRP6 proteins have been shown previously to be activated by diacylglycerol when expressed heterologously in animal cells [Hofmann, Obukhov, Schaefer, Harteneck, Gudermann and Schultz (1999) Nature (London) 397, 259-263]. In both Jurkat and peripheral blood T-lymphocytes, mRNA encoding TRP proteins 1, 3, 4 and 6 was detected by reverse transcriptase PCR, and the TRP6 protein was detected by Western blotting in a purified plasma-membrane fraction. We conclude that T-cells express a diacylglycerol-activated cation channel, unrelated to the channel involved in capacitative Ca(2+) entry, and associated with the expression of TRP6 protein.

Calcium influx factor from cytochrome P-450 metabolism and secretion-like coupling mechanisms for capacitative calcium entry in corneal endothelial cells

Notwithstanding extensive efforts, the mechanism of capacitative calcium entry (CCE) remains unclear. Two seemingly opposed theories have been proposed: secretion-like coupling (Patterson, R. L., van Rossum, D. B., and Gill, D. L. (1999) Cell 98, 487-499) and the calcium influx factor (CIF) (Randriamampita, C., and Tsien, R. Y. (1993) Nature 364, 809-814). In the current study, a combinatorial approach was taken to investigate the mechanism of CCE in corneal endothelial cells. Induction of cytochrome P-450s by beta-naphthoflavone (BN) enhanced CCE measured by Sr(2+) entry after store depletion. 5,6-Epoxyeicosatrienoic acid (5,6-EET), a proposed CIF generated by cytochrome P-450s (Rzigalinski, B. A., Willoughby, K. A., Hoffman, S. W., Falck, J. R., and Ellis, E. F. (1999) J. Biol. Chem. 274, 175-182), induced Ca(2+) entry. Both BN-enhanced CCE and the 5,6-EET-induced Ca(2+) entry were inhibited by the CCE blocker 2-aminoethoxydiphenyl borate, indicating a role for cytochrome P-450s in CCE. Treatment with calyculin A (CalyA), which causes condensation of cortical cytoskeleton, inhibited CCE. The actin polymerization inhibitor cytochalasin D partially reversed the inhibition of CCE by CalyA, suggesting a secretion-like coupling mechanism for CCE. However, CalyA could not inhibit CCE in BN-treated cells, and 5,6-EET caused a partial activation of CCE in CalyA-treated cells. These results further support the notion that cytochrome P-450 metabolites may be CIFs. The vesicular transport inhibitor brefeldin A inhibited CCE in both vehicle- and BN-treated cells. Surprisingly, Sr(2+) entry in the absence of store depletion was enhanced in BN-treated cells, which was also inhibited by 2-aminoethoxydiphenyl borate. An integrative model suggests that both CIF from cytochrome P-450 metabolism and secretion-like coupling mechanisms play roles in CCE in corneal endothelial cells.

The TRP family of cation channels: probing and advancing the concepts on receptor-activated calcium entry

Stimulation of membrane receptors linked to a phospholipase C and the subsequent production of the second messengers diacylglycerol and inositol-1,4,5-trisphosphate (InsP(3)) is a signaling pathway of fundamental importance in eukaryotic cells. Signaling downstream of these initial steps involves mobilization of Ca(2+) from intracellular stores and Ca(2+) influx through the plasma membrane. For this influx, several contrasting mechanisms may be responsible but particular relevance is attributed to the induction of Ca(2+) influx as consequence of depletion of intracellular calcium stores. This phenomenon (frequently named store-operated calcium entry, SOCE), in turn, may be brought about by various signals, including soluble cytosolic factors, interaction of proteins of the endoplasmic reticulum with ion channels in the plasma membrane, and a secretion-like coupling involving translocation of channels to the plasma membrane. Experimental approaches to analyze these mechanisms have been considerably advanced by the discovery of mammalian homologs of the Drosophila cation channel transient receptor potential (TRP). Some members of the TRP family can be expressed to Ca(2+)-permeable channels that enable SOCE; other members form channels activated independently of stores. TRP proteins may be an essential part of endogenous Ca(2+) entry channels but so far expression of most TRP cDNAs has not resulted in restitution of channels found in any mammalian cells, suggesting the requirement for further unknown subunits. A major exception is CaT1, a TRP channel demonstrated to provide Ca(2+)-selective, store-operated currents identical to those characterized in several cell types. Ongoing and future research on TRP channels will be crucial to understand the molecular basis of receptor-mediated Ca(2+) entry, with respect to the structure of the entry channels as well as to the mechanisms of its activation and regulation.