Anoctamin 1/TMEM16A in pruritoceptors is essential for Mas-related G protein receptor-dependent itch.

ABSTRACT: Itch is an unpleasant sensation that evokes a desire to scratch. Pathologic conditions such as allergy or atopic dermatitis produce severe itching sensation. Mas-related G protein receptors (Mrgprs) are receptors for many endogenous pruritogens. However, signaling pathways downstream to these receptors in dorsal root ganglion (DRG) neurons are not yet understood. We found that anoctamin 1 (ANO1), a Ca 2+ -activated chloride channel, is a transduction channel mediating Mrgpr-dependent itch signals. Genetic ablation of Ano1 in DRG neurons displayed a significant reduction in scratching behaviors in response to acute and chronic Mrgpr-dependent itch models and the epidermal hyperplasia induced by dry skin. In vivo Ca 2+ imaging and electrophysiological recording revealed that chloroquine and other agonists of Mrgprs excited DRG neurons via ANO1. More importantly, the overexpression of Ano1 in DRG neurons of Ano1 -deficient mice rescued the impaired itching observed in Ano1 -deficient mice. These results demonstrate that ANO1 mediates the Mrgpr-dependent itch signaling in pruriceptors and provides clues to treating pathologic itch syndromes.

The Piezo2 ion channel is mechanically activated by low-threshold positive pressure.

Recent parallel studies clearly indicated that Merkel cells and the mechanosensitive piezo2 ion channel play critical roles in the light-touch somatosensation. Moreover, piezo2 was suggested to be a light-touch sensing ion channel without a role in pain sensing in mammals. However, biophysical characteristics of piezo2, such as single channel conductance and sensitivities to various mechanical stimuli, are unclear, hampering a precise understanding of its role in touch sensation. Here, we describe the biophysical properties of piezo2 in human Merkel cell carcinoma (MCC)-13 cells; piezo2 is a low-threshold, positive pressure-specific, curvature-sensitive, mechanically activated cation channel with a single channel conductance of ~28.6 pS. Application of step indentations under the whole-cell mode of the patch-clamp technique, and positive pressures ≥5 mmHg under the cell-attached mode, activated piezo2 currents in MCC-13 and human embryonic kidney 293 T cells where piezo2 was overexpressed. By contrast, application of a negative pressure failed to activate piezo2 in these cells, whereas both positive and negative pressure activated piezo1 in a similar manner. Our results are the first to demonstrate single channel recordings of piezo2. We anticipate that our findings will be a starting point for a more sophisticated understanding of piezo2 roles in light-touch sensation.

Protons inhibit anoctamin 1 by competing with calcium.

Cl(-) efflux through Ca(2+)-activated Cl(-) channels (CaCCs) in secretory epithelial cells plays a key role in the regulation of fluid secretion. The fluid and electrolyte secretion is closely related to intracellular pH. CaCCs have been known to be inhibited by intracellular acid. However, the molecular mechanism for the inhibition remains unknown. Anoctamin 1 (ANO1) is a Ca(2+)-activated Cl(-) channel that mediates numerous physiological functions including fluid secretion in secretory epithelia. However, little is known about whether ANO1 can be modulated by change of intracellular pH. Here, we demonstrate that Ca(2+)-induced activation of ANO1 and its homolog ANO2 are strongly inhibited by intracellular acid. Intracellular acid caused a rightward shift of the concentration-response curve of Ca(2+) in activating ANO1 and ANO2. To identify the location of the acid-induced inhibition, mutations were made on each of all histidine residues in cytoplasmic part of ANO1. However, none of the His-mutant showed the reduction in the acid-induced inhibition. Furthermore, mutation on Glu- or Asp-residues in the multiple acidic-amino acid regions was ineffective in blocking the acid-induced inhibition. Because the Ca(2+)-binding site of a fungal anoctamin (nhTMEM16) was uncovered by crystallography, mutagenesis was performed in this region. Surprisingly, mutations at Glu, Asp or Asn residues in the hydrophobic core that are known to be essential for Ca(2+)-induced activation of ANO1 blocked the acid-induced inhibition. These results suggest that protons interfere with Ca(2+) at the Ca(2+) binding site of ANO1. These findings provide a molecular mechanism underlying the acid-induced inhibition of ANO1, which may contribute to control fluid and electrolyte secretion in the secretory epithelia.

Two helices in the third intracellular loop determine anoctamin 1 (TMEM16A) activation by calcium.

Anoctamin 1 (ANO1)/TMEM16A is a Cl(-) channel activated by intracellular Ca(2+) mediating numerous physiological functions. However, little is known of the ANO1 activation mechanism by Ca(2+). Here, we demonstrate that two helices, "reference" and "Ca(2+) sensor" helices in the third intracellular loop face each other with opposite charges. The two helices interact directly in a Ca(2+)-dependent manner. Positively and negatively charged residues in the two helices are essential for Ca(2+)-dependent activation because neutralization of these charges change the Ca(2+) sensitivity. We now predict that the Ca(2+) sensor helix attaches to the reference helix in the resting state, and as intracellular Ca(2+) rises, Ca(2+) acts on the sensor helix, which repels it from the reference helix. This Ca(2+)-dependent push-pull conformational change would be a key electromechanical movement for gating the ANO1 channel. Because chemical activation of ANO1 is viewed as an alternative means of rescuing cystic fibrosis, understanding its gating mechanism would be useful in developing novel treatments for cystic fibrosis.

Anoctamin 1 contributes to inflammatory and nerve-injury induced hypersensitivity.

BACKGROUND: Various pathological conditions such as inflammation or injury can evoke pain hypersensitivity. That represents the response to innocuous stimuli or exaggerated response to noxious stimuli. The molecular mechanism based on the pain hypersensitivity is associated with changes in many of ion channels in dorsal-root ganglion (DRG) neurons. Anoctamin 1 (ANO1/TMEM16A), a Ca2+ activated chloride channel is highly visible in small DRG neurons and responds to heat. Mice with an abolished function of ANO1 in DRG neurons demonstrated attenuated pain-like behaviors when exposed to noxious heat, suggesting a role in acute thermal nociception. In this study, we further examined the function of ANO1 in mediating inflammation- or injury-induced hyperalgesia or allodynia. RESULTS: Using Advillin/Ano1fl/fl (Adv/Ano1fl/fl) mice that have a functional ablation of Ano1 mainly in DRG neurons, we were able to determine its role in mediating thermal hyperalgesia and mechanical allodynia induced by inflammation or nerve injury. The thermal hyperalgesia and mechanical allodynia induced by carrageenan injection and spared-nerve injury were significantly reduced in Adv/Ano1fl/fl mice. In addition, flinching or licking behavior after bradykinin or formalin injection was also significantly reduced in Adv/Ano1fl/fl mice. Since pathological conditions augment nociceptive behaviors, we expected ANO1's contribution to the excitability of DRG neurons. Indeed, the application of inflammatory mediators reduced the threshold for action potential (rheobase) or time for induction of the first action potential in DRG neurons isolated from control (Ano1fl/fl) mice. These parameters for neuronal excitability induced by inflammatory mediators were not changed in Adv/Ano1fl/fl mice, suggesting an active contribution of ANO1 in augmenting the neuronal excitability. CONCLUSIONS: In addition to ANO1's role in mediating acute thermal pain as a heat sensor, ANO1 is also capable of augmenting the excitability of DRG neurons under inflammatory or neuropathic conditions and thereby aggravates inflammation- or tissue injury-induced pathological pain.

Voluntary movements as a possible non-reflexive pain assay.

BACKGROUND: The quantification of pain intensity in vivo is essential for identifying the mechanisms of various types of pain or for evaluating the effects of different analgesics. A variety of behavioral tests for pain measurement have been devised, but many are limited because animals are physically restricted, which affects pain sensation. In this study, pain assessment was attempted with minimal physical restriction, and voluntary movements of unrestrained animals were used to evaluate the intensities of various types of pain. RESULTS: The number of times animals reared or total distances traveled was measured using a motion-tracking device and found to be markedly reduced in carrageenan-induced inflammatory, acetic acid-induced visceral, and streptozotocin-induced neuropathic pain tests. These two voluntary movement parameters were found to be highly correlated with paw withdrawal latency from irradiating heat. In addition, these parameters were markedly reversed by morphine and by non-steroidal anti-inflammatory drugs in inflammatory pain models. These parameters were also useful to detect hypoalgesia in TRPV1⁻/⁻ mice. CONCLUSIONS: These results suggest that parameters of voluntary movement, such as, number of rearing and total distance moved, are effective indicators of pain intensity for many types of pain and that they can be used to evaluate degree of pain perception.

Anoctamin 1 mediates thermal pain as a heat sensor.

Vertebrates can sense and avoid noxious heat that evokes pain. Many thermoTRP channels are associated with temperature sensation. TRPV1 is a representative ion channel that is activated by noxious heat. Anoctamin 1 (ANO1) is a Cl- channel activated by calcium that is highly expressed in small sensory neurons, colocalized with markers for nociceptors, and most surprisingly, activated by noxious heat over 44oC. Although ANO1 is a Cl- channel, opening of this channel leads to depolarization of sensory neurons, suggesting a role in nociception. Indeed, the functional deletion of ANO1 in sensory neurons triggers the reduction in thermal pain sensation. Thus, it seems clear that ANO1 is a heat sensor in a nociceptive pathway. Since ANO1 modulators are developed for the purpose of treating chronic diseases such as cystic fibrosis, this finding is likely to predict unwanted effects and provide a guide for better developmental strategy.

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 Ca(2+)-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.

Axonal neuropathy-associated TRPV4 regulates neurotrophic factor-derived axonal growth.

Spinal muscular atrophy and hereditary motor and sensory neuropathies are characterized by muscle weakness and atrophy caused by the degenerations of peripheral motor and sensory nerves. Recent advances in genetics have resulted in the identification of missense mutations in TRPV4 in patients with these hereditary neuropathies. Neurodegeneration caused by Ca(2+) overload due to the gain-of-function mutation of TRPV4 was suggested as the molecular mechanism for the neuropathies. Despite the importance of TRPV4 mutations in causing neuropathies, the precise role of TRPV4 in the sensory/motor neurons is unknown. Here, we report that TRPV4 mediates neurotrophic factor-derived neuritogenesis in developing peripheral neurons. TRPV4 was found to be highly expressed in sensory and spinal motor neurons in early development as well as in the adult, and the overexpression or chemical activation of TRPV4 was found to promote neuritogenesis in sensory neurons as well as PC12 cells, whereas its knockdown and pharmacologic inhibition had the opposite effect. More importantly, nerve growth factor or cAMP treatment up-regulated the expression of phospholipase A(2) and TRPV4. Neurotrophic factor-derived neuritogenesis appears to be regulated by the phospholipase A(2)-mediated TRPV4 pathway. These findings show that TRPV4 mediates neurotrophic factor-induced neuritogenesis in developing peripheral nerves. Because neurotrophic factors are essential for the maintenance of peripheral nerves, these findings suggest that aberrant TRPV4 activity may lead to some types of pathology of sensory and motor nerves.

Isopentenyl pyrophosphate is a novel antinociceptive substance that inhibits TRPV3 and TRPA1 ion channels.

Transient receptor potential ion channels (TRPs) expressed in the periphery sense and electrically transduce noxious stimuli to transmit the signals to the brain. Many natural and synthetic ligands for the sensory TRPs have been found, but little is known about endogenous inhibitors of these TRP channels. Recently, we reported that farnesyl pyrophosphate, an endogenous substance produced in the mevalonate pathway, is a specific activator for TRPV3. Here, we show that isopentenyl pyrophosphate (IPP), an upstream metabolite in the same pathway, is a dual inhibitor for TRPA1 and TRPV3. By using Ca(2+) imaging and voltage clamp experiments with human embryo kidney cell heterologous expression system, cultured sensory neurons, and epidermal keratinocytes, we demonstrate that micromolar IPP suppressed responses to specific agonists of TRPA1 and TRPV3. Consistently, peripheral IPP administration attenuated TRPA1 and TRPV3 agonist-specific acute pain behaviors. Furthermore, local IPP pretreatment significantly reversed mechanical and thermal hypersensitivity of inflamed animals. Taken together, the present study suggests that IPP is a novel endogenous TRPA1 and TRPV3 inhibitor that causes local antinociception. Our results may provide useful chemical information to elucidate TRP physiology in peripheral pain sensation.

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.

TMEM16A confers receptor-activated calcium-dependent chloride conductance.

Calcium (Ca(2+))-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 Ca(2+)-activated chloride channel that is activated by intracellular Ca(2+) and Ca(2+)-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 Ca(2+)-activated chloride currents. ANO1 is expressed in various secretory epithelia, the retina and sensory neurons. Furthermore, knockdown of mouse Ano1 markedly reduced native Ca(2+)-activated chloride currents as well as saliva production in mice. We conclude that ANO1 is a candidate Ca(2+)-activated chloride channel that mediates receptor-activated chloride currents in diverse physiological processes.

A tarantula spider toxin, GsMTx4, reduces mechanical and neuropathic pain.

Mechanosensitive channels mediate various physiological functions including somatic sensation or pain. One of the peptide toxins isolated from the venom of the Chilean rose tarantula spider (Grammostola spatulata), mechanotoxin 4 (GsMTx4) is known to block stretch-activated cation channels. Since mechanosensitive channels in sensory neurons are thought to be molecular sensors for mechanotransduction, i.e., for touch, pressure, proprioception, and pain, we considered that the venom might block some types of mechanical pain. In order to prepare sufficiently large amounts of GsMTx4 for in vivo nociceptive behavioral tests, we constructed recombinant peptide of GsMTx4. Because the amino-acid sequence of the toxin, but not the nucleotide sequence, is known, we back-translated its amino-acid sequence to nucleotide sequence of yeast codons, constructed a template DNA, subcloned this into a Pichia pastoris expression vector, and purified the recombinant peptide. Intraperitoneal injection of the recombinant GsMTx4 to rats significantly increased the mechanical threshold for paw withdrawal in Randall Sellito test, eliciting significant analgesic responses to inflammation-induced mechanical hyperalgesia. GsMTx4 also reduced mechanical allodynia induced by inflammation and by sciatic nerve injury in Von Frey test. However, the venom was ineffective at changing withdrawal latency in hot plate and tail-flick tests. These results suggest that GsMTx4 selectively alleviates mechanical hyperalgesia, which it presumably achieves by blocking mechanosensitive channels. Because the peptide venom induces analgesia for some forms of mechanical pain, GsMTx4 appears to have potential clinical use as a pain treatment.

Hydroxy-alpha-sanshool activates TRPV1 and TRPA1 in sensory neurons.

Sanshools are major active ingredients of Zanthoxylum piperitum and are used as food additives in East Asia. Sanshools cause irritant, tingling and sometimes paresthetic sensations on the tongue. However, the molecular mechanism underlying the pungent or tingling sensation induced by sanshools is not known. Because many transient receptor potential (TRP) channels are responsible for the sensations induced by various spices and food additives, we expressed 17 TRP channels in human embryonic kidney (HEK) cells and investigated their activation by hydroxy-alpha-sanshool (HalphaSS) or hydroxy-beta-sanshool (HbetaSS) isolated from Zanthoxylum piperitum. It was found that HalphaSS, but not HbetaSS, depolarized sensory neurons with concomitant firing of action potentials and evoked inward currents. Among 17 TRP channels expressed in HEK cells, HalphaSS caused Ca(2+) influx in cells transfected with TRPV1 or TRPA1, and evoked robust inward currents in cells transfected with TRPV1 or TRPA1. In primary cultured sensory neurons, HalphaSS induced inward currents and Ca(2+) influx in a capsazepine-dependent manner. Moreover, HalphaSS-induced currents and Ca(2+) influx were greatly diminished in TRPV1(-/-) mice. HalphaSS evoked licking behavior when injected into a single hind paw of wild-type mice, but this was much reduced in TRPV1-deficient mice. These results indicate that TRPV1 and TRPA1 are molecular targets of HalphaSS in sensory neurons. We conclude that the activations of TRPV1 and TRPA1 by HalphaSS explain its unique pungent, tingling sensation.

A novel mechanosensitive channel identified in sensory neurons.

Mechanosensitive (MS) channels are ion channels gated by different types of mechanical stimuli. MS channels in sensory neurons are thought to be molecular transducers for somatic sensations such as touch, pressure, proprioception and pain. Previously, we reported that two types of MS channels are present in sensory neurons. These channels are termed low threshold (LT) and high threshold (HT) MS channels based on their pressure threshold for activation. Here, we report another type of MS channel present in sensory neurons. The channel is activated by low pressure applied to a patch (threshold approximately 20 mmHg, similar to that in the LT channel). However, because this channel has a smaller single-channel conductance than that of the LT channel, the newly classified MS channel is now called a low threshold small conductance (LTSC) channel. Unlike the LT channel, which has outwardly rectifying currents, the current-voltage relationship of the LTSC is linear. The LTSC was permeable to monovalent cations and Ca2+, and reversibly blocked by gadolinium, a blocker of MS channels. Unlike the LT channel, the LTSC was sensitized by prostaglandin E2, an inflammatory mediator that is known to sensitize nociceptors to mechanical stimuli. LTSC channels were found mostly in small cultured sensory neurons. Thus, these results suggest that the LTSC is a distinct type of MS channel that is different from the LT and HT channels in sensory neurons, and that LTSCs might play a role in mediating somatosensations, including pain.

TRPV1 recapitulates native capsaicin receptor in sensory neurons in association with Fas-associated factor 1.

TRPV1, a cloned capsaicin receptor, is a molecular sensor for detecting adverse stimuli and a key element for inflammatory nociception and represents biophysical properties of native channel. However, there seems to be a marked difference between TRPV1 and native capsaicin receptors in the pharmacological response profiles to vanilloids or acid. One plausible explanation for this overt discrepancy is the presence of regulatory proteins associated with TRPV1. Here, we identify Fas-associated factor 1 (FAF1) as a regulatory factor, which is coexpressed with and binds to TRPV1 in sensory neurons. When expressed heterologously, FAF1 reduces the responses of TRPV1 to capsaicin, acid, and heat, to the pharmacological level of native capsaicin receptor in sensory neurons. Furthermore, silencing FAF1 by RNA interference augments capsaicin-sensitive current in native sensory neurons. We therefore conclude that FAF1 forms an integral component of the vanilloid receptor complex and that it constitutively modulates the sensitivity of TRPV1 to various noxious stimuli in sensory neurons.

Biarylcarboxybenzamide derivatives as potent vanilloid receptor (VR1) antagonistic ligands.

Seventeen biarylcarboxybenzamide derivatives were prepared for the study of their agonistic/antagonistic activities to the vanilloid receptor (VR1) in rat DRG neurons. The replacement of the piperazine moiety of the lead compound 1 with phenyl ring showed quite enhanced antagonistic activity. Among the prepared derivatives, N-(4-tert-butylphenyl)-4-pyridine-2-yl-benzamide (2, IC(50)=31 nM) and N-(4-tert-butylphenyl)-4-(3-methylpyridine-2-yl)benzamide (3g, IC(50)=31 nM), showed 5-fold higher antagonistic activity than 1 in (45)Ca(2+)-influx assay.

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 Johnston's 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.

N-4-methansulfonamidobenzyl-N'-2-substituted-4-tert-butyl-benzyl thioureas as potent vanilloid receptor antagonistic ligands.

A series of N-4-methansulfonamidobenzyl-N'-2-substituted-4-tert-butylbenzyl thioureas were prepared for the study of their agonistic/antagonistic activities to the vanilloid receptor in rat DRG neurons. Their structure-activity relationship reveals that there is a space for another hydrophobic binding interaction around 2-position in 4-tert-butylbenzyl region. Among the prepared derivatives, 6n show the highest antagonistic activity against the vanilloid receptor (IC(50)=15 nM).

N-4-Substituted-benzyl-N'-tert-butylbenzyl thioureas as vanilloid receptor ligands: investigation on the role of methanesulfonamido group in antagonistic activity.

A series of N-4-substituted-benzyl-N'-tert-butylbenzyl thioureas were prepared for the study of their agonistic/antagonistic activities to the vanilloid receptor in rat DRG neurons. Their structure-activity relationship reveals that not only the two oxygens and amide hydrogen of sulfonamido group, but also the optimal size of methyl in methanesulfonamido group play an integral role for the antagonistic activity on vanilloid receptor.