Ca2+ waves in keratinocytes are transmitted to sensory neurons: the involvement of extracellular ATP and P2Y2 receptor activation

Fast calcium waves

Calcium waves are propagated in five main speed ranges which cover a billion-fold range of speeds. We define the fast speed range as 3-30μm/s after correction to a standard temperature of 20°C. Only waves which are not fertilization waves are considered here. 181 such cases are listed here. These are through organisms in all major taxa from cyanobacteria through mammals including human beings except for those through other bacteria, higher plants and fungi. Nearly two-thirds of these speeds lie between 12 and 24μm/s. We argue that their common mechanism in eukaryotes is a reaction-diffusion one involving calcium-induced calcium release, in which calcium waves are propagated along the endoplasmic reticulum. We propose that the gliding movements of some cyanobacteria are driven by fast calcium waves which are propagated along their plasma membranes. Fast calcium waves may drive materials to one end of developing embryos by cellular peristalsis, help coordinate complex cell movements during development and underlie brain injury waves. Moreover, we continue to argue that such waves greatly increase the likelihood that chronic injuries will initiate tumors and cancers before genetic damage occurs. Finally we propose numerous further studies.

Peripheral purinergic receptor modulation influences the trigeminal ganglia nitroxidergic system in an experimental murine model of inflammatory orofacial pain

ATP plays an important role as an endogenous pain mediator generating and/or modulating pain signaling from the periphery to the central nervous system. The aim of this study was to analyze the role of peripheral purinergic receptors in modulation of the nitroxidergic system at a trigeminal ganglia level by monitoring changes in nitric oxide synthase isoforms. We also evaluated Fos-positive neurons in brainstem (spinal trigeminal nucleus) and pain-related behavior. We found that local administration of the P2 purinergic receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) decreased face-rubbing activity, nitric oxide synthase isoform expression in trigeminal ganglia, and Fos expression in spinal trigeminal nucleus after subcutaneous injection of formalin. These results suggest a role for peripheral P2 purinergic receptors in orofacial pain transmission through modulation of the nitroxidergic system. .

ATP signalling is crucial for the response of human keratinocytes to mechanical stimulation by hypo-osmotic shock

Touch is detected through receptors located in the skin and the activation of channels in sensory nerve fibres. Epidermal keratinocytes themselves, however, may sense mechanical stimulus and contribute to skin sensation. Here, we showed that the mechanical stimulation of human keratinocytes by hypo-osmotic shock releases adenosine triphosphate (ATP) and increases intracellular calcium. We demonstrated that the release of ATP was found to be calcium independent because emptying the intracellular calcium stores did not cause ATP release; ATP release was still observed in the absence of external calcium and it persisted on chelating cytosolic calcium. On the other hand, the released ATP activated purinergic receptors and mobilized intracellular calcium stores. The resulting depletion of stored calcium led to the activation of capacitative calcium entry. Increase in cytosolic calcium concentration was blocked by the purinergic receptor blocker suramin, phospholipase C inhibitor and apyrase, which hydrolyses ATP. Collectively, our data demonstrate that human keratinocytes are mechanically activated by hypo-osmotic shock, leading first to the release of ATP, which in turn stimulates purinergic receptors, resulting in the mobilization of intracellular calcium and capacitative calcium entry. These results emphasize the crucial role of ATP signalling in the transduction of mechanical stimuli in human keratinocytes.

Voltage- and temperature-dependent activation of TRPV3 channels is potentiated by receptor-mediated PI(4,5)P2 hydrolysis

TRPV3 is a thermosensitive channel that is robustly expressed in skin keratinocytes and activated by innocuous thermal heating, membrane depolarization, and chemical agonists such as 2-aminoethyoxy diphenylborinate, carvacrol, and camphor. TRPV3 modulates sensory thermotransduction, hair growth, and susceptibility to dermatitis in rodents, but the molecular mechanisms responsible for controlling TRPV3 channel activity in keratinocytes remain elusive. We show here that receptor-mediated breakdown of the membrane lipid phosphatidylinositol (4,5) bisphosphate (PI(4,5)P₂) regulates the activity of both native TRPV3 channels in primary human skin keratinocytes and expressed TRPV3 in a HEK-293–derived cell line stably expressing muscarinic M₁-type acetylcholine receptors. Stimulation of PI(4,5)P₂ hydrolysis or pharmacological inhibition of PI 4 kinase to block PI(4,5)P₂ synthesis potentiates TRPV3 currents by causing a negative shift in the voltage dependence of channel opening, increasing the proportion of voltage-independent current and causing thermal activation to occur at cooler temperatures. The activity of single TRPV3 channels in excised patches is potentiated by PI(4,5)P₂ depletion and selectively decreased by PI(4,5)P₂ compared with related phosphatidylinositol phosphates. Neutralizing mutations of basic residues in the TRP domain abrogate the effect of PI(4,5)P₂ on channel function, suggesting that PI(4,5)P₂ directly interacts with a specific protein motif to reduce TRPV3 channel open probability. PI(4,5)P₂-dependent modulation of TRPV3 activity represents an attractive mechanism for acute regulation of keratinocyte signaling cascades that control cell proliferation and the release of autocrine and paracrine factors.

Chemosensory properties of the trigeminal system

The capacity of cutaneous, including trigeminal endings, to detect chemicals is known as chemesthesis or cutaneous chemosensation. This sensory function involves the activation of nociceptor and thermoreceptor endings and has a protective or defensive function, as many of these substances are irritants or poisonous. However, humans have also developed a liking for the distinct sharpness or pungency of many foods, beverages, and spices following activation of the same sensory afferents. Our understanding of the cellular and molecular mechanisms of chemosensation in the trigeminal system has experienced enormous progress in the past decade, following the cloning and functional characterization of several ion channels activated by physical and chemical stimuli. This brief review attempts to summarize our current knowledge in this field, including a functional description of various sensory channels, especially TRP channels, involved in trigeminal chemosensitivy. Finally, some of these new findings are discussed in the context of the pathophysiology of trigeminal chemosensation, including pain, pruritus, migraine, cough, airway inflammation, and ophthalmic diseases.

Pain-relieving prospects for adenosine receptors and ectonucleotidases

Adenosine receptor agonists have potent antinociceptive effects in diverse preclinical models of chronic pain. By contrast, the efficacy of adenosine and adenosine receptor agonists in treating pain in humans is unclear. Two ectonucleotidases that generate adenosine in nociceptive neurons were recently identified. When injected spinally, these enzymes have long-lasting adenosine A(1) receptor-dependent antinociceptive effects in inflammatory and neuropathic pain models. Furthermore, recent findings indicate that spinal adenosine A(2A) receptor activation can enduringly inhibit neuropathic pain symptoms. Collectively, these studies suggest the possibility of treating chronic pain in humans by targeting specific adenosine receptor subtypes in anatomically defined regions with agonists or with ectonucleotidases that generate adenosine.

Involvement of P2Y6 receptor in p38 MAPK-mediated COX-2 expression in response to UVB irradiation of human keratinocytes

Ultraviolet B (UVB) radiation induces inflammation in human skin. Extracellular nucleotides are released from cells in response to various stimuli and act as intercellular signaling molecules through activation of P2 receptors. In this study, we investigated the involvement of extracellular nucleotides and P2 receptors in UVB-radiation-induced inflammation using human keratinocyte-derived HaCaT cells. UVB radiation induced rapid ATP release from HaCaT cells; this was inhibited by pretreatment with anion transporter blockers or maxi-anion channel blockers. In addition, the radiation-induced activation of p38 MAPK was significantly blocked by pretreatment with ecto-nucleotidase (apyrase) or P2Y6 receptor antagonist (MRS2578). Expression of COX-2, mediated by activation of p38 MAPK, was also induced by UVB radiation. Both pretreatment with MRS2578 and knockdown of the P2Y6 receptor by siRNA transfection attenuated the induction of COX-2 in HaCaT cells exposed to UVB radiation. Our results indicate that UVB radiation evokes ATP release from human keratinocytes and also that activation of P2Y6 receptor mediates the UVB-radiation-induced activation of p38 MAPK and expression of COX-2. Thus P2Y6 receptor is a mediator of UVB-radiation-induced inflammatory responses in keratinocytes.

Dynamic plasticity of axons within a cutaneous milieu

The skin is a repository of sensory axons immersed within the turnover of epidermal, follicular, and dermal cellular constituents. We show that epidermal and perifollicular axons within intact hairy skin of mice possess a remarkable dynamic plasticity linked to their microenvironment. For example, the majority of epidermal axons express the growth protein GAP43. Unexpectedly, we induced new cutaneous axogenesis by simple and noninvasive hair clipping, a response linked to a series of changes in their cutaneous neighbors. In thy-1 YFP transgenic mice with fluorescent axons, superficial epidermal and perifollicular cells newly acquired YFP, indicating diffuse activation by clipping despite the absence of skin injury. At 48 h after clipping, this activation was accompanied by a rise in the number of epidermal cells, transient rises in mRNA of Sox2, a marker of follicular stem cells, and a rise in mRNA of glial fibrillary acidic protein, a marker of glial cells. Axons responded with rises in their numbers in the epidermis and around dermal hair follicles. Linking these responses were early, large, and selective rises in hepatic growth factor (HGF) mRNA, with its protein identified in epidermal cells, perifollicular cells, and sensory axons. Moreover, these elements also expressed the HGF receptor c-Met, especially in small caliber sensory neurons. Finally, we identified concurrent rises in Rac1 activation, a downstream target of ligated c-Met. Together, these results confirm critical linkages between sensory axons and their cutaneous milieu. We believe that the plasticity is provoked by follicular-originating cutaneous activation with HGF and Rac1 signaling, allowing cross talk and axonal remodeling.

Involvement of peripheral adenosine 5'-triphosphate and P2X purinoceptor in pain-related behavior produced by orthotopic melanoma inoculation in mice

Adenosine 5'-triphosphate (ATP) plays an important role in nociceptive processing. We used a mouse model of skin cancer pain to investigate the role of ATP in cancer pain. Orthotopic inoculation of B16-BL6 melanoma cells into the hind paw produced spontaneous licking of the tumor-bearing paw. Intraperitoneal injection of the P2 purinoceptor antagonist suramin suppressed spontaneous licking dose-dependently. Two P2X purinoceptor antagonists also suppressed spontaneous licking. An intraplantar injection of ATP, which did not induce licking in the healthy paw, increased licking of the tumor-bearing paw. Spontaneous firing of the tibial nerve was significantly increased in tumor-bearing mice and was inhibited by suramin. Extracellular concentration of ATP was significantly increased in the tumor-bearing paw than in the normal paw. ATP is concentrated in the culture medium of melanoma, lung cancer and breast cancer cells, but not fibroblasts. The P2X(3) receptor was expressed in about 40% of peripherin-positive small and medium-sized neurons in the dorsal root ganglia. P2X(3)-positive neurons were significantly increased in melanoma-bearing mice. These results suggest that ATP and P2X, especially P2X(3), receptors are involved in skin cancer pain, due to the increased release of ATP and increased expression of P2X(3) receptors in the sensory neurons.

Ultra-low-level laser therapy

A growing number of laboratory and clinical studies over the past 10 years have shown that low-level laser stimulation (633 or 670 nm) at extremely low power densities (about 0.15 mW/cm(2)), when administered through a particular emission mode, is capable of eliciting significant biological effects. Studies on cell cultures and animal models as well as clinical trials give support to a novel therapeutic modality, which may be referred to as ultra low level laser therapy (ULLLT). In cultured neural cells, pulsed irradiation (670 nm, 0.45 mJ/cm(2)) has shown to stimulate NGF-induced neurite elongation and to protect cells against oxidative stress. In rats, anti-edema and anti-hyperalgesia effects following ULLL irradiation were found. Clinical studies have reported beneficial effects (also revealed through sonography) in the treatment of musculoskeletal disorders. The present paper reviews the existing experimental evidence available on ULLLT. Furthermore, the puzzling issue of the biophysical mechanisms that lie at the basis of the method is explored and some hypotheses are proposed. Besides presenting the state-of-the-art about this novel photobiostimulation therapy, the present paper aims to open up an interdisciplinary discussion and stimulate new research on this subject.

Calcium ion propagation in cultured keratinocytes and other cells in skin in response to hydraulic pressure stimulation

We have previously suggested that a variety of environmental factors might be first sensed by epidermal keratinocytes, which represent the frontier of the body. To further examine this idea, in the present study, we examined the intracellular calcium responses of cultured keratinocytes to external hydraulic pressure. First, we compared the responses of undifferentiated and differentiated keratinocytes with those of fibroblasts, vascular endothelial cells (VEC), and lymphatic endothelial cells. Elevation of intracellular calcium was observed after application of pressure to keratinocytes, fibroblasts, and VEC. The calcium propagation extended over a larger area and continued for a longer period of time in differentiated keratinocytes, as compared with the other cells. The response of the keratinocytes was dramatically reduced when the cells were incubated in medium without calcium. Application of a non-selective transient receptor potential (TRP) channel blocker also attenuated the calcium response. These results suggest that differentiated keratinocytes are sensitive to external pressure and that TRP might be involved in the mechanism of their response.

The P2Y2 receptor mediates the epithelial injury response and cell migration

Injury to epithelial cells results in the release of ATP and stimulation of purinergic receptors and is thought to alter cell migration and wound repair. Medium from the injured cells triggers Ca(2+) mobilization and phosphorylation of ERK, both of which are inhibited if the medium is pretreated with apyrase. To understand the wound repair mechanism that occurs with injury, our goal was to determine which purinergic receptor(s) was the critical player in the wound response. We hypothesize that the P2Y(2) receptor is the key player in the response of corneal epithelial cells to cell damage and subsequent repair events. Cells transfected with short interfering RNA to either P2Y(2) or P2Y(4) were stimulated either by injury or addition of UTP and imaged using fluo 3-AM to monitor changes in fluorescence. When cells with downregulated P2Y(2) receptors were injured or stimulated with UTP, the intensity of the Ca(2+) release was reduced significantly. However, when cells with downregulated P2Y(4) receptors were stimulated, only the UTP-induced Ca(2+) response was reduced significantly. In addition, downregulation of the P2Y(2) receptor inhibited wound closure compared with unstimulated cells or cells transfected with nontargeting sequence. This downregulation resulted also in an attenuation in phosphorylation of Src and ERK. Together, these data indicate that the P2Y(2) receptor plays a major biological role in the corneal injury response and repair mechanisms.

Farnesyl pyrophosphate is a novel pain-producing molecule via specific activation of TRPV3

Temperature-sensitive transient receptor potential ion channels (thermoTRPs) expressed in epidermal keratinocytes and sensory afferents play an important role as peripheral pain detectors for our body. Many natural and synthetic compounds have been found to act on the thermoTRPs leading to altered nociception, but little is known about endogenous painful molecules activating TRPV3. Here, we show that farnesyl pyrophosphate (FPP), an intermediate metabolite in the mevalonate pathway, specifically activates TRPV3 among six thermoTRPs using Ca(2+) imaging and electrophysiology with cultured keratinocytes and TRPV3-overexpressing cells. Agonistic potencies of related compounds in the FPP metabolism were ignorable. Voltage-dependence of TRPV3 was shifted by FPP, which appears to be the activation mechanism. An intraplantar injection of FPP acutely elicits nociceptive behaviors in inflamed animals, indicating that FPP is a novel endogenous pain-producing substance via TRPV3 activation. Co-culture experiments demonstrated that this FPP-evoked signal in the keratinocytes is transmitted to sensory neurons. In addition, FPP reduced TRPV3 heat threshold resulting in heightened behavioral sensitivity to noxious heat. Taken together, our data suggest that FPP is the firstly identified endogenous TRPV3 activator that causes nociception. Our results may provide useful chemical information to elucidate TRPV3 physiology and novel pain-related metabolisms.

Involvement of purinergic signaling in cellular response to gamma radiation

Recent studies have suggested a bystander effect in nonirradiated cells adjacent to irradiated cells; however, the mechanism is poorly understood. In this study, we investigated the involvement of both extracellular nucleotides and activation of P2 receptors in cellular responses to gamma radiation using human HaCaT keratinocytes. The concentration of ATP in culture medium was increased after gamma irradiation (0.1-1.0 Gy), suggesting that radiation induces ATP release from cells. Intracellular Ca(2+) concentration was elevated when conditioned medium from irradiated cells was transferred to nonirradiated cells, and this elevation was suppressed by apyrase (ecto-nucleotidase), indicating the involvement of extracellular nucleotides in this event. Further, we examined the activation of ERK1/2 by gamma radiation and nucleotides (ATP and UTP). Both gamma radiation and nucleotides induced activation of ERK1/2. Next, the effect of inhibitors of P2 receptors on radiation-induced activation of ERK1/2 was examined. The activation of ERK1/2 was blocked by suramin (P2Y inhibitor), MRS2578 (P2Y(6) antagonist) and apyrase. These results suggest that both released nucleotides and activation of P2Y receptors are involved in gamma-radiation-induced activation of ERK1/2. We conclude that ionizing radiation induces release of nucleotides from cells, leading to activation of P2Y receptors, which in turn would result in a variety of biological effects.

Peripheral acid-sensing ion channels and P2X receptors contribute to mechanical allodynia in a rodent thrombus-induced ischemic pain model

We have previously established a thrombus-induced ischemic pain (TIIP) model in the rat, which mimics the pathophysiology of ischemic pain in patients with peripheral arterial disease. Because ischemia commonly induces acidosis and ATP release, one of the goals of this study was to investigate the role of acid-sensing ion channels (ASICs), transient receptor potential vanilloid-1 (TRPV1) receptors, and P2X receptors in the maintenance of ischemia-induced mechanical allodynia (MA). To test this, amiloride (an ASIC blocker), AMG-9810 (a TRPV1 blocker), or PPADS (a P2Xs antagonist) was intraplantarly injected at day 3 after FeCl(2) application onto the femoral artery. Ipsilateral administration of amiloride or PPADS but not AMG-9810 dose-dependently reduced MA. However, contralateral amiloride or PPADS did not suppress contralateral MA. Interestingly, co-administration of submaximal doses of amiloride and PPADS produced a significantly prolonged suppression of MA. Furthermore, ipsilateral EGTA (a calcium chelator) or chelerythrine (a protein kinase C inhibitor) also significantly reduced MA. Collectively, these findings suggest that peripheral ASICs and P2X receptors are involved in the maintenance of TIIP, which is possibly mediated by a Ca(2+)-protein kinase C signaling mechanism. These results provide mechanistic information about peripheral ischemic nociception that may be useful for developing better therapeutic management of ischemic pain in patients with peripheral arterial disease.

PERSPECTIVE

The results of the current study demonstrate that peripheral administration of an ASICs blocker or P2X antagonist significantly suppress TIIP. Co-administration of submaximal doses of ASIC and P2X antagonists produced an even greater effect. These results implicate peripheral ASICs and P2X receptors in the maintenance of thrombus-induced ischemic pain.

Physiological basis of tingling paresthesia evoked by hydroxy-alpha-sanshool

Hydroxy-alpha-sanshool, the active ingredient in plants of the prickly ash plant family, induces robust tingling paresthesia by activating a subset of somatosensory neurons. However, the subtypes and physiological function of sanshool-sensitive neurons remain unknown. Here we use the ex vivo skin-nerve preparation to examine the pattern and intensity with which the sensory terminals of cutaneous neurons respond to hydroxy-alpha-sanshool. We found that sanshool excites virtually all D-hair afferents, a distinct subset of ultrasensitive light-touch receptors in the skin and targets novel populations of Abeta and C fiber nerve afferents. Thus, sanshool provides a novel pharmacological tool for discriminating functional subtypes of cutaneous mechanoreceptors. The identification of sanshool-sensitive fibers represents an essential first step in identifying the cellular and molecular mechanisms underlying tingling paresthesia that accompanies peripheral neuropathy and injury.

Ecto-5'-nucleotidase (CD73) inhibits nociception by hydrolyzing AMP to adenosine in nociceptive circuits

Ecto-5'-nucleotidase (NT5E, CD73) is a membrane-anchored protein that hydrolyzes extracellular adenosine 5'-monophosphate (AMP) to adenosine in diverse tissues but has not been directly studied in nociceptive neurons. We found that NT5E was located on peptidergic and nonpeptidergic nociceptive neurons in dorsal root ganglia (DRG) and on axon terminals in lamina II (the substantia gelatinosa) of spinal cord. NT5E was also located on epidermal keratinocytes, cells of the dermis, and on nociceptive axon terminals in the epidermis. Following nerve injury, NT5E protein and AMP histochemical staining were coordinately reduced in lamina II. In addition, AMP hydrolytic activity was reduced in DRG neurons and spinal cord of Nt5e(-/-) mice. The antinociceptive effects of AMP, when combined with the adenosine kinase inhibitor 5-iodotubericidin, were reduced by approximately 50% in Nt5e(-/-) mice and were eliminated in Adenosine A(1) receptor (A(1)R, Adora1) knock-out mice. Additionally, Nt5e(-/-) mice displayed enhanced sensitivity in the tail immersion assay, in the complete Freund's adjuvant model of inflammatory pain and in the spared nerve injury model of neuropathic pain. Collectively, our data indicate that the ectonucleotidase NT5E regulates nociception by hydrolyzing AMP to adenosine in nociceptive circuits and represents a new molecular target for the treatment of chronic pain. Moreover, our data suggest NT5E is well localized to regulate nucleotide signaling between skin cells and sensory axons.

Gene expression profiling defines the role of ATP-exposed keratinocytes in skin inflammation

BACKGROUND

Various environmental stimuli, e.g., mechanical stress, osmolarity change, oxidative stress, and microbial products trigger ATP release from cells. It is well known that ATP regulates cell growth, differentiation, terminal differentiation, and cell-to-cell communication in keratinocytes. Moreover, extracellular ATP stimulates the expression and release of IL-6 and modulates the production several chemokines by keratinocytes.

OBJECTIVE

To investigate the role of ATP-stimulated keratinocytes in skin inflammation and immune response.

METHODS

We identified genes whose expression is augmented in ATP-stimulated human keratinocytes by DNA microarray. These microarray data were validated by quantitative real-time RT-PCR. Furthermore, we confirmed the observed mRNA change at protein level by ELISA and Western blotting.

RESULTS

The statistical analysis of the microarray data revealed that, besides IL-6, the expression of several novel genes such as IL-20, CXCL1-3, and ATF3 was significantly augmented in ATP-stimulated keratinocytes. These data was validated by quantitative real-time RT-PCR. We also confirmed the augmented production of IL-6, IL-20, CXCL1 by ELISA and that of ATF3 by Western blotting. Since both IL-6 and IL-20 that can stimulate STAT3 were produced by the ATP-stimulated keratinocytes, we examined their phosphorylation of STAT3. The study demonstrated biphasic activation of STAT3 after ATP stimulation, which was composed of a first peak at 1-2 h and a second peak at 12-24 h. The latter peak was significantly suppressed by anti-IL-6 antibody.

CONCLUSION

These studies characterized (1) STAT3 activation, (2) chemotaxis for neutrophils via CXCL1-3, and (3) ATF3 activation as possible roles of ATP-stimulated keratinocytes in skin inflammation and immune response.

Gq-coupled purinergic receptors inhibit insulin-like growth factor-I/phosphoinositide 3-kinase pathway-dependent keratinocyte migration

After skin wound, released growth factors and extracellular nucleotides regulate the different phases of healing, including re-epithelialization. Here, we show that, in keratinocytes, purinergic P2Y2 receptors inhibit the motogenic IGF-I/PI3K pathway. Therefore, extracellular nucleotides may play key roles during skin remodelling after wound.

Insulin-like growth factor-I (IGF-I) activation of phosphoinositol 3-kinase (PI3K) is an essential pathway for keratinocyte migration that is required for epidermis wound healing. We have previously reported that activation of Gα_(q/11)-coupled-P2Y₂ purinergic receptors by extracellular nucleotides delays keratinocyte wound closure. Here, we report that activation of P2Y₂ receptors by extracellular UTP inhibits the IGF-I–induced p110α-PI3K activation. Using siRNA and pharmacological inhibitors, we demonstrate that the UTP antagonistic effects on PI3K pathway are mediated by Gα_(q/11)—and not G_(i/o)—independently of phospholipase Cβ. Purinergic signaling does not affect the formation of the IGF-I receptor/insulin receptor substrate-I/p85 complex, but blocks the activity of a membrane-targeted active p110α mutant, indicating that UTP acts downstream of PI3K membrane recruitment. UTP was also found to efficiently attenuate, within few minutes, the IGF-I–induced PI3K-controlled translocation of the actin-nucleating protein cortactin to the plasma membrane. This supports the UTP ability to alter later migratory events. Indeed, UTP inhibits keratinocyte spreading and migration promoted by either IGF-I or a membrane-targeted active p110α mutant, in a Gα(q/11)-dependent manner both. These findings provide new insight into the signaling cross-talk between receptor tyrosine kinase and Gα_(q/11)-coupled receptors, which mediate opposite effects on p110α-PI3K activity and keratinocyte migration.

P2 receptor-mediated signaling in mast cell biology

Mast cells are widely recognized as effector cells of allergic inflammatory reactions. They contribute to the pathogenesis of different chronic inflammatory diseases, wound healing, fibrosis, thrombosis/fibrinolysis, and anti-tumor immune responses. In this paper, we summarized the role of P2X and P2Y receptors in mast cell activation and effector functions. Mast cells are an abundant source of ATP which is stored in their granules and secreted upon activation. We discuss the contribution of mast cells to the extracellular ATP release and to the maintenance of extracellular nucleotides pool. Recent publications highlight the importance of purinergic signaling for the pathogenesis of chronic airway inflammation. Therefore, the role of ATP and P2 receptors in allergic inflammation with focus on mast cells was analyzed. Finally, ATP functions as mast cell autocrine/paracrine factor and as messenger in intercellular communication between mast cells, nerves, and glia in the central nervous system.

Purines and sensory nerves

P2X and P2Y nucleotide receptors are described on sensory neurons and their peripheral and central terminals in dorsal root, nodose, trigeminal, petrosal, retinal and enteric ganglia. Peripheral terminals are activated by ATP released from local cells by mechanical deformation, hypoxia or various local agents in the carotid body, lung, gut, bladder, inner ear, eye, nasal organ, taste buds, skin, muscle and joints mediating reflex responses and nociception. Purinergic receptors on fibres in the dorsal spinal cord and brain stem are involved in reflex control of visceral and cardiovascular activity, as well as relaying nociceptive impulses to pain centres. Purinergic mechanisms are enhanced in inflammatory conditions and may be involved in migraine, pain, diseases of the special senses, bladder and gut, and the possibility that they are also implicated in arthritis, respiratory disorders and some central nervous system disorders is discussed. Finally, the development and evolution of purinergic sensory mechanisms are considered.

Modeling and analysis of the molecular basis of pain in sensory neurons

Intracellular calcium dynamics are critical to cellular functions like pain transmission. Extracellular ATP plays an important role in modulating intracellular calcium levels by interacting with the P2 family of surface receptors. In this study, we developed a mechanistic mathematical model of ATP-induced P2 mediated calcium signaling in archetype sensory neurons. The model architecture, which described 90 species connected by 162 interactions, was formulated by aggregating disparate molecular modules from literature. Unlike previous models, only mass action kinetics were used to describe the rate of molecular interactions. Thus, the majority of the 252 unknown model parameters were either association, dissociation or catalytic rate constants. Model parameters were estimated from nine independent data sets taken from multiple laboratories. The training data consisted of both dynamic and steady-state measurements. However, because of the complexity of the calcium network, we were unable to estimate unique model parameters. Instead, we estimated a family or ensemble of probable parameter sets using a multi-objective thermal ensemble method. Each member of the ensemble met an error criterion and was located along or near the optimal trade-off surface between the individual training data sets. The model quantitatively reproduced experimental measurements from dorsal root ganglion neurons as a function of extracellular ATP forcing. Hypothesized architecture linking phosphoinositide regulation with P2X receptor activity explained the inhibition of P2X-mediated current flow by activated metabotropic P2Y receptors. Sensitivity analysis using individual and the whole system outputs suggested which molecular subsystems were most important following P2 activation. Taken together, modeling and analysis of ATP-induced P2 mediated calcium signaling generated qualitative insight into the critical interactions controlling ATP induced calcium dynamics. Understanding these critical interactions may prove useful for the design of the next generation of molecular pain management strategies.

TRPV3 in keratinocytes transmits temperature information to sensory neurons via ATP

Transient receptor potential V3 (TRPV3) and TRPV4 are heat-activated cation channels expressed in keratinocytes. It has been proposed that heat-activation of TRPV3 and/or TRPV4 in the skin may release diffusible molecules which would then activate termini of neighboring dorsal root ganglion (DRG) neurons. Here we show that adenosine triphosphate (ATP) is such a candidate molecule released from keratinocytes upon heating in the co-culture systems. Using TRPV1-deficient DRG neurons, we found that increase in cytosolic Ca²⁺-concentration in DRG neurons upon heating was observed only when neurons were co-cultured with keratinocytes, and this increase was blocked by P2 purinoreceptor antagonists, PPADS and suramin. In a co-culture of keratinocytes with HEK293 cells (transfected with P2X₂ cDNA to serve as a bio-sensor), we observed that heat-activated keratinocytes secretes ATP, and that ATP release is compromised in keratinocytes from TRPV3-deficient mice. This study provides evidence that ATP is a messenger molecule for mainly TRPV3-mediated thermotransduction in skin.

Mechanical-stimulation-evoked calcium waves in proliferating and differentiated human keratinocytes

Calcium dynamics in the epidermis play a crucial role in barrier homeostasis and keratinocyte differentiation. We have recently suggested that the electro-physiological responses of the keratinocyte represent the frontier of the skin sensory system for environmental stimuli. In the present study, we have evaluated the responses of proliferating and differentiated human keratinocytes to mechanical stress by measuring the intracellular calcium level. Before differentiation, mechanical stress induces a calcium wave over a limited area; this is completely blocked by apyrase, which degrades ATP. In the case of differentiated keratinocytes, the calcium wave propagates over a larger area. Application of apyrase does not completely inhibit this wave. Thus, in differentiated cells, the induction of calcium waves might involve not only ATP, but also another factor. Immunohistochemical studies indicate that connexins 26 and 43, both components of gap junctions, are expressed in the cell membrane of differentiated keratinocytes. Application of octanol or carbenxolone, which block gap junctions, significantly reduces calcium wave propagation in differentiated keratinocytes. Thus, signaling via gap junctions might be involved in the induction of calcium waves in response to mechanical stress at the upper layer of the epidermis.

Mammalian somatosensory mechanotransduction

In the mammalian somatosensory system, mechanosensitive neurons mediate the senses of touch and pain. Among sensory modalities, mechanosensation has been the most elusive with regard to the identification of transduction molecules. One factor that has hindered the identification of transduction molecules is the diversity of neurons; physiological studies have revealed many subtypes of neurons, specialized to detect a variety of mechanical stimuli. Do different subtypes use the same transduction molecules that are modified by cellular context? Or, are there multiple mechanotransducers that specialize in sensing different mechanical stimuli? This review highlights recent progress in identifying and characterizing candidate molecular force transducers, as well as the development of new tools to characterize touch transduction at the molecular, cellular, and behavioral levels.

Expression of transient receptor potential vanilloid (TRPV) 1, 2, 3, and 4 in mouse inner ear

CONCLUSION

It is suggested that transient receptor potential vanilloids (TRPVs) may play a functional role in cell physiology and TRPV-4 and -2 may play an important part in fluid homeostasis in the inner ear.

OBJECTIVE

Expression of TRPV-1, -2, -3, and -4 in the normal mouse inner ear was studied.

MATERIALS AND METHODS

CBA/J mice were used in this study. The localization of TRPV-1, -2, -3, and -4 in the inner ear, i.e. cochlea, vestibular end organs, and endolymphatic sac, was investigated by immunohistochemistry.

RESULTS

TRPV-1, -2, and -3 were co-expressed in hair cells and supporting cells of the organ of Corti, in spiral ganglion cells, sensory cells in vestibular end organs, vestibular ganglion cells, and sensory nerve fibers. TRPV-2 was also detected in the stria vascularis, dark cells, and endolymphatic sac. TRPV-4 was expressed in hair cells and supporting cells of the organ of Corti, in marginal cells of the stria vascularis, spiral ganglion cells, vestibular sensory cells, vestibular dark cells, vestibular ganglion cells, and epithelial cells of the endolymphatic sac.

The TRPV4 cation channel mediates stretch-evoked Ca2+ influx and ATP release in primary urothelial cell cultures

Transient receptor potential channels have recently been implicated in physiological functions in a urogenital system. In this study, we investigated the role of transient receptor potential vanilloid 4 (TRPV4) channels in a stretch sensing mechanism in mouse primary urothelial cell cultures. The selective TRPV4 agonist, 4alpha-phorbol 12,13-didecanoate (4alpha-PDD) evoked Ca(2+) influx in wild-type (WT) urothelial cells, but not in TRPV4-deficient (TRPV4KO) cells. We established a cell-stretch system to investigate stretch-evoked changes in intracellular Ca(2+) concentration and ATP release. Stretch stimulation evoked intracellular Ca(2+) increases in a stretch speed- and distance-dependent manner in WT and TRPV4KO cells. In TRPV4KO urothelial cells, however, the intracellular Ca(2+) increase in response to stretch stimulation was significantly attenuated compared with that in WT cells. Stretch-evoked Ca(2+) increases in WT urothelium were partially reduced in the presence of ruthenium red, a broad TRP channel blocker, whereas that in TRPV4KO cells did not show such reduction. Potent ATP release occurred following stretch stimulation or 4alpha-PDD administration in WT urothelial cells, which was dramatically suppressed in TRPV4KO cells. Stretch-dependent ATP release was almost completely eliminated in the presence of ruthenium red or in the absence of extracellular Ca(2+). These results suggest that TRPV4 senses distension of the bladder urothelium, which is converted to an ATP signal in the micturition reflex pathway during urine storage.

Localization of transient receptor potential vanilloid (TRPV) in the human larynx

CONCLUSION

Transient receptor potential vanilloid (TRPV) 1, 2, 3, and 4 were expressed in the human larynx, which may act as laryngeal nociceptors perceiving luminal noxious stimuli, play an important role in thermal sensation and osmotic sensation, and are also related to some pathological conditions and prevention of aspiration.

OBJECTIVE

Expression of TRPV1, 2, 3, and 4 in the human larynx was analyzed.

MATERIALS AND METHODS

Specimens of human epiglottic epithelium obtained from six patients were used in this study. The localization of TRPV1, 2, 3, and 4 in the laryngeal epithelium was investigated by immunohistochemistry.

RESULTS

Immunohistochemical study revealed the presence of TRPV1, 2, 3, and 4 in the laryngeal epithelial cells. Chemoradiotherapy may reduce the expression of TRPV1, 2, 3, and 4, which might be a result of the mucositis and neuropathy in laryngeal epithelium.

TPRV1 expression defines functionally distinct pelvic colon afferents

Changes in primary sensory neurons are likely to contribute to the emergence of chronic visceral pain. An important step in understanding visceral pain is the development of comprehensive phenotypes that combines functional and anatomical properties for these neurons. We developed a novel ex vivo physiology preparation in mice that allows intracellular recording from colon sensory neurons during colon distension, in the presence and absence of pharmacologic agents. This preparation also allows recovery of functionally characterized afferents for histochemical analysis. Recordings obtained from L6 dorsal root ganglion cells in C57BL/6 mice identified two distinct populations of distension-responsive colon afferents: high-firing frequency (HF) and low-firing frequency (LF) cells. Fluid distension of the colon elicited rapid firing (>20 Hz) in HF cells, whereas LF cells seldom fired >5 Hz. Distension response thresholds were significantly lower in HF cells (LF, 17.5 +/- 1.1 cmH(2)O; HF, 2.6 +/- 1.0 cmH(2)O). Responses of most LF afferents to colon distension were sensitized by luminal application of capsaicin (1 microm; 8 of 9 LF cells), mustard oil (100 microm; 10 of 12 LF cells), and low pH (pH 4.0; 5 of 6 LF cells). In contrast, few HF afferents were sensitized by capsaicin (3 of 9), mustard oil (2 of 7), or low pH (1 of 6) application. Few HF afferents (4 of 23) expressed the capsaicin receptor, TRPV1. In contrast, 87% (25 of 29) of LF afferents expressed TRPV1. TRPV1 has been shown to be required for development of inflammatory hyperalgesia. These results suggest a unique functional role of TRPV1-positive colon afferents that could be exploited to design specific therapies for visceral hypersensitivity.

Overexpressed transient receptor potential vanilloid 3 ion channels in skin keratinocytes modulate pain sensitivity via prostaglandin E2

The ability to sense changes in the environment is essential for survival because it permits responses such as withdrawal from noxious stimuli and regulation of body temperature. Keratinocytes, which occupy much of the skin epidermis, are situated at the interface between the external environment and the body's internal milieu, and have long been appreciated for their barrier function against external insults. The recent discovery of temperature-sensitive transient receptor potential vanilloid (TRPV) ion channels in keratinocytes has raised the possibility that these cells also actively participate in acute temperature and pain sensation. To address this notion, we generated and characterized transgenic mice that overexpress TRPV3 in epidermal keratinocytes under the control of the keratin 14 promoter. Compared with wild-type controls, keratinocytes overexpressing TRPV3 exhibited larger currents as well as augmented prostaglandin E(2) (PGE(2)) release in response to two TRPV3 agonists, 2-aminoethoxydiphenyl borate (2APB) and heat. Thermal selection behavior and heat-evoked withdrawal behavior of naive mice overexpressing TRPV3 were not consistently altered. Upon selective pharmacological inhibition of TRPV1 with JNJ-17203212 [corrected], however, the keratinocyte-specific TRPV3 transgenic mice showed increased escape responses to noxious heat relative to their wild-type littermates. Coadministration of the cyclooxygenase inhibitor, ibuprofen, with the TRPV1 antagonist decreased inflammatory thermal hyperalgesia in transgenic but not wild-type animals. Our results reveal a previously undescribed mechanism for keratinocyte participation in thermal pain transduction through keratinocyte TRPV3 ion channels and the intercellular messenger PGE(2).

Nucleotide signaling and cutaneous mechanisms of pain transduction

Sensory neurons that innervate the skin provide critical information about physical contact between the organism and the environment, including information about potentially-damaging stimuli that give rise to the sensation of pain. These afferents also contribute to the maintenance of tissue homeostasis, inflammation and wound healing, while sensitization of sensory afferents after injury results in painful hypersensitivity and protective behavior. In contrast to the traditional view of primary afferent terminals as the sole site of sensory transduction, recent reports have lead to the intriguing idea that cells of the skin play an active role in the transduction of sensory stimuli. The search for molecules that transduce different types of sensory stimuli (mechanical, heat, chemical) at the axon terminal has yielded a wide range of potential effectors, many of which are expressed by keratinocytes as well as neurons. Emerging evidence underscores the importance of nucleotide signaling through P2X ionotropic and P2Y metabotropic receptors in pain processing, and implicates nucleotide signaling as a critical form of communication between cells of the skin, immune cells and sensory neurons. It is of great interest to determine whether pathological changes in these mechanisms contribute to chronic pain in human disease states such as complex regional pain syndrome (CRPS). This review discusses recent advances in our understanding of communication mechanisms between cells of the skin and sensory axons in the transduction of sensory input leading to pain.

Calcium ion gradients and dynamics in cultured skin slices of rat hindpaw in response to stimulation with ATP

Ionotropic receptors, originally found in the brain, were recently also identified in epidermal keratinocytes. Moreover, concentration gradients and movement of calcium are crucial in epidermal homeostasis. Thus, imaging of calcium in the living epidermis is expected to provide insight into epidermal physiology and pathology. Here we describe the imaging of calcium dynamics in the living epidermis of cultured skin slices. The basal calcium concentration was highest in the upper layer of the epidermis. The increase of intracellular calcium in response to adenosine triphosphate (ATP) varied in each layer of epidermis, and was greater at the bottom than in the uppermost layer. Further, the extent of elevation of intracellular calcium in response to ATP in cultured keratinocytes varied depending on the level of differentiation. These results suggest that the response to stimulation of keratinocytes in cultured skin slices varies depending upon the location (depth) within the epidermis.

Sustained depolarization-induced propagation of [Ca2+]i oscillations in cultured DRG neurons: the involvement of extracellular ATP and P2Y receptor activation

Recently emerging evidence implicates a number of neuroactive substances and their receptors in mediating complex cell-to-cell communications in the ganglia. In the present study, we characterized the nonsynaptic chemical coupling mediated by extracellular ATP in dorsal root ganglia (DRG) neuron cultures by using the real time imaging of ATP, whole-cell patch clamping, in conjunction with confocal calcium imaging. Sustained depolarization by electrical stimulation evoked intracellular Ca2+ concentrations ([Ca2+]i) oscillations in individual DRG neurons, and subsequent ATP-dependent propagation [Ca2+]i oscillations to surrounding non-stimulated neighbors. [Ca2+]i oscillations were suppressed by inositol-1,4,5-trisphosphate (IP3) receptor antagonist 2-APB, but not ryanodine. The propagation of [Ca2+]i oscillations was prevented by the presence of the ATP-degrading enzyme, apyrase, and completely abolished by the blockase of G protein-coupled purinergic receptors-PLC-IP3 pathway with suramin, U73122 or 2-APB. In parallel, sustained depolarization elicited robust ATP release and diffusion from the stimulation site. Moreover, exogenous application of ATP to DRG cultures in large concentration elicits the [Ca2+]i oscillations in most neurons. Taken together, this data demonstrates that sustained membrane depolarization elicited ATP release, acting through a highly sensitive P2Y receptors/IP3-mediated signaling pathway to mediate the propagation of intercellular Ca2+ signaling, which suggest a novel signaling pathway for neuronal communication in DRG.

Compression-induced ATP release from rat skeletal muscle with and without lengthening contraction

Adenosine triphosphate (ATP) is well known to be released from injured or inflamed tissues, and to excite/sensitize nociceptors in response to heat and mechanical stimulation. To determine whether muscle releases ATP when it is compressed, we measured ATP release from the extensor digitorum longus muscle (EDL). In addition, we investigated whether there is any difference in ATP release from the EDL of rats 2 days after lengthening contraction (LC), since the condition of the muscle is different, i.e., mechanically hyperalgesic and swollen. The EDL was put in a small chamber and superfused with Krebs-Henseleit solution equilibrated with a gas mixture of 95% oxygen and 5% carbon dioxide. The muscle was quantitatively stimulated with a servo-controlled mechanical stimulator. Reproducibility of ATP release was examined with stimulation using a 20 g force. Stimulus intensity-dependency of ATP release was also examined with 5 time compression with intensities of 5, 10, 20 and 40 g force. Bioluminescent determination by the luciferin-luciferase method was used to quantify ATP in the sample. The ATP release was decreased by repetitive mechanical stimulation of the EDL with 30 min intervals, and it was stimulus intensity (5-40 g force)-dependent. The amount of ATP released from the muscle preparations was not different between the non-treated control and the LC group. These results provide clear evidence that ATP is released from rat skeletal muscle by compression.

Cutaneous sensory neurons expressing the Mrgprd receptor sense extracellular ATP and are putative nociceptors

Sensory neurons expressing the Mrgprd receptor are known to innervate the outermost living layer of the epidermis, the stratum granulosum. The sensory modality that these neurons signal and the stimulus that they respond to are not established, although immunocytochemical data suggest they could be nonpeptidergic nociceptors. Using patch clamp of dissociated mouse dorsal root ganglion (DRG) neurons, the present study demonstrates that Mrgprd+ neurons have several properties typical of nociceptors: long-duration action potentials, TTX-resistant Na(+) current, and Ca(2+) currents that are inhibited by mu opioids. Remarkably, Mrgprd+ neurons respond almost exclusively to extracellular ATP with currents similar to homomeric P2X3 receptors. They show little or no sensitivity to other putative nociceptive agonists, including capsaicin, cinnamaldehyde, menthol, pH 6.0, or glutamate. These properties, together with selective innervation of the stratum granulosum, indicate that Mrgprd+ neurons are nociceptors in the outer epidermis and may respond indirectly to external stimuli by detecting ATP release in the skin.

Recent progress in sensory mechanism

Pain serves as a warning of impending injury, triggering appropriate protective responses. Emotional and cognitive processing in the brain is involved in the sensation of pain. As Ca(2+) waves in keratinocytes are mediated by the release of extracellular molecules such as signaling molecules, this may also affect the activity of surrounding cells such as sensory neurons. Although no junctions have been found between keratinocytes and sensory termini, ultrastructural studies have shown that keratinocytes come into contact with dorsal root ganglion neurons through membrane-membrane apposition. There is also indirect evidence that keratinocytes communicate with sensory neurons via extracellular molecules. Sensory neurons themselves sense various external stimuli, but there may also be skin-derived regulatory mechanisms by which sensory signaling is modulated.First, we will give a general outline of the subject: 1) Progress in identifying cortical loci that process pain messages is needed. 2) Far greater advances have been made in understanding the molecular mechanisms whereby primary sensory neurons detect pain-producing stimuli. 3) Genetic studies have facilitated the identification and functional characterization of molecules. 4) Now, the relationship between sensory and ion channels has become clear.

Inhibition of calcium-independent phospholipase A impairs agonist-induced calcium entry in keratinocytes

Background

In many cells, depletion of intracellular calcium (Ca²⁺) reservoirs triggers Ca²⁺ entry through store-operated Ca²⁺ channels in the plasma membrane. However, the mechanisms of agonist-induced calcium entry (ACE) in keratinocytes are not fully understood.

Objectives

This study was designed to determine if pharmacological inhibition of calcium-independent phospholipase A (iPLA₂) impairs ACE in normal human epidermal keratinocytes.

Methods

Confocal laser scanning microscopy was used to monitor the dynamics of Ca²⁺ signalling in keratinocytes loaded with the calcium-sensitive dye Fluo-4. Cells were stimulated with extracellular nucleotides [adenosine triphosphate (ATP) or uridine triphosphate (UTP)] or with lysophosphatidic acid (LPA), a bioactive lipid that regulates keratinocyte proliferation and differentiation.

Results

Both ATP and UTP induced Ca²⁺ release in primary human keratinocytes. This was not followed by robust Ca²⁺ influx when the experiments were performed in low Ca²⁺ (70 μmol L⁻¹) medium. Upon elevation of extracellular Ca²⁺ to 1·2 mmol L⁻¹, however, a biphasic response consisting of an initial Ca²⁺ peak followed by an elevated plateau was observed. The plateau phase was inhibited when cells were treated with bromoenol lactone, a specific pharmacological inhibitor of iPLA₂. These findings indicate that iPLA₂ activity is required for ACE in keratinocytes. LPA also evoked Ca²⁺ release in keratinocytes but failed to induce sustained Ca²⁺ entry even when extracellular Ca²⁺ was elevated to 1·2 mmol L⁻¹.

Conclusion

Our results demonstrate for the first time an important role for iPLA₂ in regulating ACE in primary human keratinocytes.

ATP plays a role in neurite stimulation with activated mast cells

Previously, we showed that nerve-mast cell cross-talk can occur bidirectionally and that substance P is a mediator to activate mast cells. Here, we have studied the mediators to activate nerves cocultured with mast cells. Addition of antigen to the cocultures of superior cervical ganglia (SCG) and rat basophilic leukemia cells (RBLs) elicited Ca(2+) response in RBLs and after a lag period induced Ca(2+) signal in SCG neurites. Pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (purinergic receptor antagonist) or apyrase (ATP-hydrolyzing enzyme) reduced the Ca(2+) signals in neurites, indicating that ATP released from activated mast cells was one of important mediators to activate nerves.