Uhtaek Oh

TMEM16A confers receptor-activated calcium-dependent chloride conductance.

Calcium (Ca2+)-activated chloride channels are fundamental mediators in numerous physiological processes including transepithelial secretion, cardiac and neuronal excitation, sensory transduction, smooth muscle contraction and fertilization. Despite their physiological importance, their molecular identity has remained largely unknown. Here we show that transmembrane protein 16A (TMEM16A, which we also call anoctamin 1 (ANO1)) is a bona fide Ca2+-activated chloride channel that is activated by intracellular Ca2+ and Ca2+-mobilizing stimuli. With eight putative transmembrane domains and no apparent similarity to previously characterized channels, ANO1 defines a new family of ionic channels. The biophysical properties as well as the pharmacological profile of ANO1 are in full agreement with native Ca2+-activated chloride currents. ANO1 is expressed in various secretory epithelia, the retina and sensory neurons. Furthermore, knockdown of mouse Ano1 markedly reduced native Ca2+-activated chloride currents as well as saliva production in mice. We conclude that ANO1 is a candidate Ca2+-activated chloride channel that mediates receptor-activated chloride currents in diverse physiological processes.

Bradykinin-12-lipoxygenase-VR1 signaling pathway for inflammatory hyperalgesia

The capsaicin-sensitive vanilloid receptor (VR1) was recently shown to play an important role in inflammatory pain (hyperalgesia), but the underlying mechanism is unknown. We hypothesized that pain-producing inflammatory mediators activate capsaicin receptors by inducing the production of fatty acid agonists of VR1. This study demonstrates that bradykinin, acting at B2 bradykinin receptors, excites sensory nerve endings by activating capsaicin receptors via production of 12-lipoxygenase metabolites of arachidonic acid. This finding identifies a mechanism that might be targeted in the development of new therapeutic strategies for the treatment of inflammatory pain.

TRPV1 Mediates Histamine-Induced Itching via the Activation of Phospholipase A2 and 12-Lipoxygenase

Histamine provokes itching and is a major skin disease complaint. Histamine is known to excite a subset of sensory neurons, predominantly C-fibers. Although histamine is pruritogenic, its signaling pathways that excite sensory neurons have not been identified. Because the metabolic products of lipoxygenases (LOs) activate transient receptor potential vanilloid receptor-1 (TRPV1) in sensory neurons, we hypothesized that histamine excites sensory neurons by activating TRPV1 via phospholipase A2 (PLA2) and LO stimulation. In cultured sensory neurons, histamine evoked inward currents that were reduced by capsazepine, a TRPV1 blocker. Moreover, histamine provoked inward currents when histamine receptor subtype 1 (H1R) and TRPV1 were expressed heterologously, but not when H1R or TRPV1 was expressed alone. In addition, histamine caused Ca2+ influxes in sensory neurons in wild-type mice but not in TRPV1−/− mice. Furthermore, histamine caused a 2.5-fold increase in the production of 12-hydroxyeicosatetraenoic acid, a metabolite of LO, in cultured sensory neurons. When injected subcutaneously into the necks of mice, histamine caused bouts of scratching, which were greatly reduced by pretreatment with capsazepine, a TRPV1 blocker, and by inhibitors of PLA2, LO, and H1R. Furthermore, mice lacking TRPV1 markedly reduced histamine-induced scratching compared with wild type. Together, these results indicate that TRPV1 plays a key role in mediating the pruritogenic action of histamine via the PLA2/LO pathway.

Two Interdependent TRPV Channel Subunits, Inactive and Nanchung, Mediate Hearing in Drosophila

Hearing in Drosophila depends on the transduction of antennal vibration into receptor potentials by ciliated sensory neurons in Johnstons organ, the antennal chordotonal organ. We previously found that a Drosophila protein in the vanilloid receptor subfamily (TRPV) channel subunit, Nanchung (NAN), is localized to the chordotonal cilia and required to generate sound-evoked potentials (Kim et al., 2003). Here, we show that the only other Drosophila TRPV protein is mutated in the behavioral mutant inactive (iav). The IAV protein forms a hypotonically activated channel when expressed in cultured cells; in flies, it is specifically expressed in the chordotonal neurons, localized to their cilia and required for hearing. IAV and NAN are each undetectable in cilia of mutants lacking the other protein, indicating that they both contribute to a heteromultimeric transduction channel in vivo. A functional green fluorescence protein-IAV fusion protein shows that the channel is restricted to the proximal cilium, constraining models for channel activation.

The calcium-activated chloride channel anoctamin 1 acts as a heat sensor in nociceptive neurons

Nociceptors are a subset of small primary afferent neurons that respond to noxious chemical, thermal and mechanical stimuli. Ion channels in nociceptors respond differently to noxious stimuli and generate electrical signals in different ways. Anoctamin 1 (ANO1 also known as TMEM16A) is a Ca2+-activated chloride channel that is essential for numerous physiological functions. We found that ANO1 was activated by temperatures over 44 °C with steep heat sensitivity. ANO1 was expressed in small sensory neurons and was highly colocalized with nociceptor markers, which suggests that it may be involved in nociception. Application of heat ramps to dorsal root ganglion (DRG) neurons elicited robust ANO1-dependent depolarization. Furthermore, knockdown or deletion of ANO1 in DRG neurons substantially reduced nociceptive behavior in thermal pain models. These results indicate that ANO1 is a heat sensor that detects nociceptive thermal stimuli in sensory neurons and possibly mediates nociception.

Peripheral mechanisms of pain and analgesia

This review summarizes recent findings on peripheral mechanisms underlying the generation and inhibition of pain. The focus is on events occurring in peripheral injured tissues that lead to the sensitization and excitation of primary afferent neurons, and on the modulation of such mechanisms. Primary afferent neurons are of particular interest from a therapeutic perspective because they are the initial generator of noxious impulses traveling towards relay stations in the spinal cord and the brain. Thus, if one finds ways to inhibit the sensitization and/or excitation of peripheral sensory neurons, subsequent central events such as wind-up, sensitization and plasticity may be prevented. Most importantly, if agents are found that selectively modulate primary afferent function and do not cross the blood-brain-barrier, centrally mediated untoward side effects of conventional analgesics (e.g. opioids, anticonvulsants) may be avoided. This article begins with the peripheral actions of opioids, turns to a discussion of the effects of adrenergic co-adjuvants, and then moves on to a discussion of pro-inflammatory mechanisms focusing on TRP channels and nerve growth factor, their signaling pathways and arising therapeutic perspectives.

A capsaicin-receptor antagonist, capsazepine, reduces inflammation-induced hyperalgesic responses in the rat: evidence for an endogenous capsaicin-like substance.

In the present study, the presence of an endogenous capsaicin-like substance and the role of capsaicin receptors in nociception during inflammation were assessed using Fos immunohistochemistry and the paw-withdrawal test in rats. Intradermal injection of carrageenan in the hind-paw produced inflammation in the foot pad, increased the number of cells exhibiting Fos-like immunoreactivity in the dorsal horn of the spinal cord, and decreased the paw-withdrawal latency. Intradermal injection of capsazepine, a capsaicin-receptor antagonist, significantly reduced the number of cells exhibiting Fos-like immunoreactivity, significantly increased the paw-withdrawal latency, but did not decrease inflammation induced by carrageenan injection. Intradermal injection of capsaicin or formalin also increased Fos-positive neurons. Capsaicin- or formalin-induced Fos expression was reduced in both cases by pretreatment of capsazepine, but to a much lesser extent for formalin. The capsazepine inhibition of carrageenan inflammation-induced hyperalgesic responses strongly suggests that an endogenous capsaicin-like substance is released in inflamed tissues and produces nociceptive neural impulses by acting on capsaicin receptors present on sensory neurons. Furthermore, our results indicate that capsaicin receptors take part only in generating nociceptive signals in sensory neurons, but not in activating the inflammation-promoting cells.

Dynamic modulation of ANO1/TMEM16A HCO3− permeability by Ca2+/calmodulin

Anoctamin 1 (ANO1)/transmembrane protein 16A (TMEM16A) is a calcium-activated anion channel that may play a role in HCO3− secretion in epithelial cells. Here, we report that the anion selectivity of ANO1 is dynamically regulated by the Ca2+/calmodulin complex. Whole-cell current measurements in HEK 293T cells indicated that ANO1 becomes highly permeable to HCO3− at high [Ca2+]i. Interestingly, this result was not observed in excised patches, indicating the involvement of cytosolic factors in this process. Further studies revealed that the direct association between ANO1 and calmodulin at high [Ca2+]i is responsible for changes in anion permeability. Calmodulin physically interacted with ANO1 in a [Ca2+]i-dependent manner, and addition of recombinant calmodulin to the cytosolic side of excised patches reversibly increased PHCO3/PCl. In addition, the high [Ca2+]i-induced increase in HCO3− permeability was reproduced in mouse submandibular gland acinar cells, in which ANO1 plays a critical role in fluid secretion. These results indicate that the HCO3− permeability of ANO1 can be dynamically modulated and that ANO1 may play an important role in cellular HCO3− transport, especially in transepithelial HCO3− secretion.

Histamine-induced Ca2+ influx via the PLA2/lipoxygenase/TRPV1 pathway in rat sensory neurons ☆

Histamine is known to excite a subset of C-fibers and cause itch sensation. Despite its well-defined excitatory action on sensory neurons, intracellular signaling mechanisms are not understood. Previously, we demonstrated that bradykinin excited sensory neurons by activating TRPV1 via the phospholipase A2 (PLA2) and lipoxygenase (LO) pathway. We, thus, hypothesized that histamine excited sensory neurons via the PLA2/LO/TRPV1 pathway. Application of histamine elicited a rapid increase in intracellular Ca2+ ([Ca2+]i) that desensitized slowly in cultured dorsal root ganglion neurons. Histamine-induced [Ca2+]i was dependent on extracellular Ca2+ and inhibited by capsazepine and by SC0030, competitive antagonists of TRPV1. Quinacrine and nordihydroguaiaretic acid, a PLA2 and an LO inhibitor, respectively, blocked the histamine-induced Ca2+ influx in sensory neurons, while indomethacin (a cyclooxygenase inhibitor) did not. We thus conclude that histamine activates TRPV1 after stimulating the PLA2/LO pathway, leading to the excitation of sensory neurons. These results further provide an idea for potential use of TRPV1 antagonists as anti-itch drugs.

A role for Piezo2 in EPAC1-dependent mechanical allodynia

Aberrant mechanosensation has an important role in different pain states. Here we show that Epac1 (cyclic AMP sensor) potentiation of Piezo2-mediated mechanotransduction contributes to mechanical allodynia. Dorsal root ganglia Epac1 mRNA levels increase during neuropathic pain, and nerve damage-induced allodynia is reduced in Epac1−/− mice. The Epac-selective cAMP analogue 8-pCPT sensitizes mechanically evoked currents in sensory neurons. Human Piezo2 produces large mechanically gated currents that are enhanced by the activation of the cAMP-sensor Epac1 or cytosolic calcium but are unaffected by protein kinase C or protein kinase A and depend on the integrity of the cytoskeleton. In vivo, 8-pCPT induces long-lasting allodynia that is prevented by the knockdown of Epac1 and attenuated by mouse Piezo2 knockdown. Piezo2 knockdown also enhanced thresholds for light touch. Finally, 8-pCPT sensitizes responses to innocuous mechanical stimuli without changing the electrical excitability of sensory fibres. These data indicate that the Epac1–Piezo2 axis has a role in the development of mechanical allodynia during neuropathic pain.