Dissecting receptor-mediated Ca2+ influx pathways: TRP channels and their native counterparts

Evaluation of osteoblastic cell behavior upon culture on titanium substrates photo-functionalized by vacuum ultra-violet treatment

Photo-functionalization of titanium orthopedic/prosthetic implants using ultraviolet illumination is known to improve osteogenesis. Therefore, in this study, we aimed to examine the influence of vacuum ultraviolet (VUV)-treated titanium surfaces on osteoblast cell adhesion, activity, and differentiation. Osteoblastic cells were cultured on titanium substrates treated with various VUV treatment conditions (0, 6.2, 18.7, and 37.4 J/cm²) and their behavior was evaluated. The results revealed that cell adhesion was increased whereas cell activity and differentiation ability were decreased upon cell culture on VUV-treated substrates. In particular, cell activity and differentiation ability were dramatically suppressed with 18.7 J/cm² VUV irradiation. Within the limitations of this cell-based experiment, we clarified the VUV treatment conditions in which cell adhesion was improved but cell activity and differentiation ability were suppressed. These results indicate that VUV-treatment can be used to influence cell growth properties and can be used to accelerate or suppress cell differentiation on implant substrates.

Activation of transient receptor potential canonical 3 (TRPC3)-mediated Ca2+ entry by A1 adenosine receptor in cardiomyocytes disturbs atrioventricular conduction

Although the activation of the A(1)-subtype of the adenosine receptors (A(1)AR) is arrhythmogenic in the developing heart, little is known about the underlying downstream mechanisms. The aim of this study was to determine to what extent the transient receptor potential canonical (TRPC) channel 3, functioning as receptor-operated channel (ROC), contributes to the A(1)AR-induced conduction disturbances. Using embryonic atrial and ventricular myocytes obtained from 4-day-old chick embryos, we found that the specific activation of A(1)AR by CCPA induced sarcolemmal Ca(2+) entry. However, A(1)AR stimulation did not induce Ca(2+) release from the sarcoplasmic reticulum. Specific blockade of TRPC3 activity by Pyr3, by a dominant negative of TRPC3 construct, or inhibition of phospholipase Cs and PKCs strongly inhibited the A(1)AR-enhanced Ca(2+) entry. Ca(2+) entry through TRPC3 was activated by the 1,2-diacylglycerol (DAG) analog OAG via PKC-independent and -dependent mechanisms in atrial and ventricular myocytes, respectively. In parallel, inhibition of the atypical PKCζ by myristoylated PKCζ pseudosubstrate inhibitor significantly decreased the A(1)AR-enhanced Ca(2+) entry in both types of myocytes. Additionally, electrocardiography showed that inhibition of TRPC3 channel suppressed transient A(1)AR-induced conduction disturbances in the embryonic heart. Our data showing that A(1)AR activation subtly mediates a proarrhythmic Ca(2+) entry through TRPC3-encoded ROC by stimulating the phospholipase C/DAG/PKC cascade provide evidence for a novel pathway whereby Ca(2+) entry and cardiac function are altered. Thus, the A(1)AR-TRPC3 axis may represent a potential therapeutic target.

Emodin down-regulates expression of TRPV1 mRNA and its function in DRG neurons in vitro

Emodin is a principle ingredient isolated from rhubarb rhizome, which is commonly used for constipation or pain-related diseases in traditional Chinese medicine (TCM) practice. The transient receptor potential vanilloid 1 ion channel proteins (TRPV1) are abundantly expressed in the peripheral sensory neurons and are assumed to act as a kind of nociceptor involved in the perception of pain and development of hyperalgesia. The aim of this study was to further unravel the analgesic mechanisms of rhubarb through investigating the effects of its main constitutive ingredient emodin on the expression of TRPV1 mRNA as well as on its calcium- mediating functions in vitro. The primary DRG neurons with a high purity and viability were obtained, and the TRPV1 mRNA expression levels were examined by using real-time RT-PCR and the elevated amplitudes of intracellular [Ca(2+)]i in the DRG neurons evoked by TRPV1 agonist capsaicin were examined by confocal microscopy. The results showed that emodin could significantly down-regulate both the mRNA expression of TRPV1 and the capsaicin-evoked intracellular fluorescent intensity in the DRG neurons under both 37 degrees C and 39 degrees C in vitro. Concomitantly, all of the changes induced by emodin could not be blocked by pretreatment of the primary neurons with capsazepine, an antagonist of TRPV1. In conclusion, we established that the mRNA expression level of TRPV1 and its calcium-mediating function in naive DRG neurons could be down-regulated by emodin through perhaps the non-TRPV1 channel pathways, and this might be the molecular mechanisms for rhubarb to inhibit hyperalgesia induced by inflammatory stimuli.

Functional requirement for Orai1 in store-operated TRPC1-STIM1 channels

Orai1 and TRPC1 have been proposed as core components of store-operated calcium release-activated calcium (CRAC) and store-operated calcium (SOC) channels, respectively. STIM1, a Ca(2+) sensor protein in the endoplasmic reticulum, interacts with and mediates store-dependent regulation of both channels. We have previously reported that dynamic association of Orai1, TRPC1, and STIM1 is involved in activation of store-operated Ca(2+) entry (SOCE) in salivary gland cells. In this study, we have assessed the molecular basis of TRPC1-SOC channels in HEK293 cells. We report that TRPC1+STIM1-dependent SOCE requires functional Orai1. Thapsigargin stimulation of cells expressing Orai1+STIM1 increased Ca(2+) entry and activated typical I(CRAC) current. STIM1 alone did not affect SOCE, whereas expression of Orai1 induced a decrease. Expression of TRPC1 induced a small increase in SOCE, which was greatly enhanced by co-expression of STIM1. Thapsigargin stimulation of cells expressing TRPC1+STIM1 activated a non-selective cation current, I(SOC), that was blocked by 1 microm Gd(3+) and 2-APB. Knockdown of Orai1 decreased endogenous SOCE as well as SOCE with TRPC1 alone. siOrai1 also significantly reduced SOCE and I(SOC) in cells expressing TRPC1+STIM1. Expression of R91WOrai1 or E106QOrai1 induced similar attenuation of TRPC1+STIM1-dependent SOCE and I(SOC), whereas expression of Orai1 with TRPC1+STIM1 resulted in SOCE that was larger than that with Orai1+STIM1 or TRPC1+STIM1 but not additive. Additionally, Orai1, E106QOrai1, and R91WOrai1 co-immunoprecipitated with similar levels of TRPC1 and STIM1 from HEK293 cells, and endogenous TRPC1, STIM1, and Orai1 were co-immunoprecipitated from salivary glands. Together, these data demonstrate a functional requirement for Orai1 in TRPC1+STIM1-dependent SOCE.

Transient Receptor Potential Channels of the Melastatin Family and Ischemic Responses of Central Neurons

The excitotoxic theory of stroke, which implicated N-methyl-d-aspartate (NMDA) receptors as mediators of excessive Ca(2+) entry and neuronal death, generated a great deal of enthusiasm for the prospect of using NMDA receptor antagonists to prevent the associated brain injury. Unfortunately, these receptor antagonists failed to provide effective treatments for human stroke. In part, the failure is likely a consequence of having to administer these drugs within a very short therapeutic window after stroke and to the intolerable psychomimetic side effects associated with their use. However, new possibilities for therapeutic intervention are revealing themselves as our understanding of excitotoxicity evolves. We now recognize that ischemia and Ca(2+) toxicity in central neurons can be attributed to a variety of mechanisms recruited downstream of NMDA receptor activation. These include the activation of Ca(2+)-permeable transient receptor potential channels of the melastatin family. The more-delayed activation of these channels offers the tantalizing possibility that drugs targeting selected members of this family may possess a wider therapeutic window for preventing the debilitating consequences after stroke onset.

Membrane Stretch-Induced Activation of a TRPM4-Like Nonselective Cation Channel in Cerebral Artery Myocytes

Stretch-activated cation channels (SACs) have been observed in many types of smooth muscle cells. However, the molecular identity and activation mechanisms of SACs remain poorly understood. We report that TRPM4-like cation channels are activated by membrane stretch in rat cerebral artery myocytes (CAMs). Negative pressure (> or =20 mmHg, cell-attached mode) activated single channels (approximately 20 pS) in isolated CAMs. These channels were permeable to Na(+) and Cs(+) and inhibited by Gd(3+) (30 microM) and DIDS (100 microM). The effect of negative pressure was abolished by membrane excision, but subsequent application of Ca(2+) (>100 nM) to the intracellular side of the membrane restored single channel activity that was indistinguishable from SACs. Caffeine (5 mM), which depletes SR Ca(2+)-stores, first activated and then abolished SACs. Tetracaine (100 microM), a ryanodine receptor antagonist, inhibited SACs. Overexpression of hTRPM4B in HEK293 cells resulted in the appearance of cation channels that were activated by both negative pressure and Ca(2+) and which had very similar biophysical and pharmacological properties as compared with SACs in CAMs. These studies indicate that TRPM4-like channels in CAMs can be activated by membrane stretch, possibly through ryanodine receptor activation, and this may contribute to the depolarization and concomitant vasoconstriction of intact cerebral arteries following mechanical stimulation.

Adrenergic modulation of the type 1 IP3 receptors in the rat heart

Inositol 1,4,5-trisphosphate (IP3) receptors are calcium-releasing channels localized on the sarcoplasmic reticulum. IP3 receptors mediate the calcium mobilizing effect of a wide range of hormones, cytokines, and neurotransmitters and play an important role in variety of cell functions. The aim of this work was to study, how partial depletion of catecholamines affects the gene expression and protein levels of the type 1 IP3 receptors in rat heart. The type 1 IP3 receptor mRNA levels were studied in the left cardiac atrium and ventricle of rats treated with 6-hydroxydopamine (6-OHDA) in control and stressed conditions. The 6-OHDA produces anatomical and functional denervation resulting in decreased levels of noradrenaline and adrenaline. We also used corticoliberin (CRH) knockout mice, where secretion of adrenaline is significantly suppressed. Administration of 6-OHDA significantly decreases mRNA levels of the type 1 IP3 receptor in both, the left atrium and the left ventricle, while the gene expression of the sarcoplasmic reticular Ca2+-ATPase (SERCA 2) was unaffected. CRH knockout mice possess markedly lower levels of the type 1 IP3 receptor mRNA compared to wild-type mice in both, control and stressed conditions. These data point to the adrenergic modulation of the type 1 IP3 receptors in the rat hearts.

Noradrenaline-induced cation currents in isolated rat paratracheal ganglion neurons

The actions of noradrenaline (NA) on the neurons acutely isolated from paratracheal ganglia of rats and the ionic mechanisms involved were studied with nystatin-perforated patch recording configuration. Under current-clamp conditions, application of 10 microM NA produced membrane depolarization followed by repetitive action potentials. NA evoked an inward cationic current under voltage-clamp conditions at a holding potential of -60 mV. Transient tail inward ('hump') current was also induced by washout of NA. The NA-induced current was reduced by extracellular Ca(2+) and Mg(2+), with half-maximal concentrations of 0.7 and 2.6 mM for Ca(2+) and Mg(2+), respectively. Phenylephrine, an alpha(1)-adrenoceptor agonist, mimicked the NA-induced current, but the 'hump' current did not occur upon washout of phenylephrine. The NA-induced current was inhibited by prazosin and WB-4101, alpha(1)-adrenoceptor antagonists. In contrast, in the presence of yohimbine, an alpha(2)-adrenoceptor antagonist, the NA-induced current was potentiated and the washout of NA failed to evoke the 'hump' current. The pretreatment of paratracheal neurons with pertussis toxin also potentiated the NA-induced current. The NA-induced inward current was inhibited by pretreatment with U73122, a phospholipase C inhibitor, and xestospongin-C, a membrane-permeable IP(3) receptor antagonist. On the other hand, thapsigargin, BAPTA-AM and calmidazolium had no effect on the NA-induced current, suggesting that release of Ca(2+) from intracellular Ca(2+) stores via IP(3) receptors is not involved in the NA action. The cationic channels activated by NA play an important physiological role in neuronal membrane depolarization in rat paratracheal ganglia.

Ca2+ store-independent augmentation of [Ca2+]i responses to G-protein coupled receptor activation in recombinantly TRPC5-expressed rat pheochromocytoma (PC12) cells

Mammalian homologues of the Drosophila canonical transient receptor potential (trp) protein (TRPC) have been implicated to function as receptor-operated Ca(2+) channels (ROCs) or store-operated Ca(2+) channels (SOCs). To determine the role of TRPC5 protein in neural cells, TRPC5 was recombinantly expressed in rat pheochromocytoma cells (PC12) and changes in intracellular Ca(2+) concentration ([Ca(2+)](i)) and Na(+) concentration ([Na(+)](i)) were analyzed. TRPC1 and TRPC3 mRNAs were endogenously expressed in PC12 cells. In TRPC5-expressed cells (TRPC5-cells), the resting [Ca(2+)](i) and [Na(+)](i) were significantly higher than those in control cells. The [Ca(2+)](i) increases induced by bradykinin and uridine 5'-triphosphate were significantly larger in TRPC5-cells. TRPC5 expression did not change in store-operated Ca(2+) entry elicited by thapsigarigin. TRPC5-cells showed larger inward current and increase of [Na(+)](i) in response to BK than control cells. These results suggest that TRPC5 channels expressed in PC12 cells function as ROCs activated by G-protein/phospholipase C coupled receptors, but not as SOCs.

Alpha 1-adrenoceptor-activated cation currents in neurones acutely isolated from rat cardiac parasympathetic ganglia

The noradrenaline (NA)-induced cation current was investigated in neurones freshly isolated from rat cardiac parasympathetic ganglia using the nystatin-perforated patch recording configuration. Under current-clamp conditions, NA depolarized the membrane, eliciting repetitive action potentials. NA evoked an inward cation current under voltage-clamp conditions at a holding potential of -60 mV. The NA-induced current was inhibited by extracellular Ca2+ or Mg2+, with a half-maximal concentration of 13 microM for Ca2+ and 1.2 mM for Mg2+. Cirazoline mimicked the NA response, and prazosin and WB-4101 inhibited the NA-induced current, suggesting the contribution of an alpha1-adrenoceptor. The NA-induced current was inhibited by U73122, a phospholipase C (PLC) inhibitor. The membrane-permeable IP3 receptor blocker xestospongin-C also blocked the NA-induced current. Furthermore, pretreatment with thapsigargin and BAPTA-AM could inhibit the NA response while KN-62, phorbol 12-myristate 13-acetate (PMA) and staurosporine had no effect. These results suggest that NA activates the extracellular Ca2+- and Mg2+-sensitive cation channels via alpha 1-adrenoceptors in neurones freshly isolated from rat cardiac parasympathetic ganglia. This activation mechanism also involves phosphoinositide breakdown, release of Ca2+ from intracellular Ca2+ stores and calmodulin. The cation channels activated by NA may play an important role in neuronal membrane depolarization in rat cardiac ganglia.

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.

[Analyses of Ca-related ion channel currents and their involvement in Ca mobilization in smooth muscle and endothelial cells]

Changes in intracellular Ca concentration ([Ca2+]i) play dominant roles in the regulation of ion channel activity. Thus, analyses of Ca-related ion channels, whose activation is responsible for and/or dependent on the changes in [Ca2+]i, are important to understand the physiological and pharmacological characteristics of smooth muscle cells (SMCs) and endothelial cells (ECs). We have clarified that, in SMCs, Ca mobilization by membrane depolarization and bioactive substances affects the activity of Ca-activated K (IK-Ca) and Cl channel currents. On the other hand, by measuring IK-Ca as an indicator of Ca mobilization, we found that palmitoylcarnitine (PC), a lipid released under ischemic conditions, mobilizes Ca in ECs via stimulation of endothelial differential gene (Edg) receptors. Moreover, sphingosine-1-phosphate, which is a lipid mediator and has a similar structure to PC, elevated [Ca2+]i in ECs via the activation of cation channels through Edg1 receptors. A myo-endothelial interaction is another regulatory factor of Ca mobilization in ECs as well as in SMCs. Nifedipine and levcromakalim, which have no effects on ion channels in ECs themselves, changed the membrane potential of ECs via a myo-endothelial pathway. These integral analyses provide better understanding of the functional roles of Ca-related ion channels and their involvement in Ca mobilization in SMCs and ECs.

A cold- and menthol-activated current in rat dorsal root ganglion neurones: properties and role in cold transduction

Skin temperature is sensed by peripheral thermoreceptors. Using the neuronal soma in primary culture as a model of the receptor terminal, we have investigated the mechanisms of cold transduction in thermoreceptive neurones from rat dorsal root ganglia. Cold-sensitive neurones were pre-selected by screening for an increase in [Ca(2+)](i) on cooling; 49 % of them were also excited by 0.5 microM capsaicin. Action potentials and voltage-gated currents of cold-sensitive neurones were clearly distinct from those of cold-insensitive neurones. All cold-sensitive neurones expressed an inward current activated by cold and sensitised by (-)-menthol, which was absent from cold-insensitive neurones. This current was carried mainly by Na(+) ions and caused a depolarisation on cooling accompanied by action potentials, inducing voltage-gated Ca(2+) entry; a minor fraction of Ca(2+) entry was voltage-independent. Application of (-)-menthol shifted the threshold temperatures of the cold-induced depolarisation and the inward current to the same extent, indicating that the cold- and menthol-activated current normally sets the threshold temperature for depolarisation during cooling. The action of menthol was stereospecific, with the (+)-isomer being a less effective agonist than the (-)-isomer. Extracellular Ca(2+) modulated the cold- and menthol-activated current in a similar way to its action on intact cold receptors: lowered [Ca(2+)](o) sensitised the current, while raised [Ca(2+)](o) antagonised the menthol-induced sensitisation. During long cooling pulses the current showed adaptation, which depended on extracellular Ca(2+) and was mediated by a rise in [Ca(2+)](i). This adaptation consisted of a shift in the temperature sensitivity of the channel. In capsaicin-sensitive neurones, capsaicin application caused a profound depression of the cold-activated current. Inclusion of nerve growth factor in the culture medium shifted the threshold of the cold-activated current towards warmer temperatures. The current was blocked by 50 microM capsazepine and 100 microM SKF 96365. We conclude that the cold- and menthol-activated current is the major mechanism responsible for cold-induced depolarisation in DRG neurones, and largely accounts for the known transduction properties of intact cold receptors.