Triggering of Major Brain Disorders by Protons and ATP: The Role of ASICs and P2X Receptors

Adenosine triphosphate (ATP) is well-known as a universal source of energy in living cells. Less known is that this molecule has a variety of important signaling functions: it activates a variety of specific metabotropic (P2Y) and ionotropic (P2X) receptors in neuronal and non-neuronal cell membranes. So, a wide variety of signaling functions well fits the ubiquitous presence of ATP in the tissues. Even more ubiquitous are protons. Apart from the unspecific interaction of protons with any protein, many physiological processes are affected by protons acting on specific ionotropic receptors-acid-sensing ion channels (ASICs). Both protons (acidification) and ATP are locally elevated in various pathological states. Using these fundamentally important molecules as agonists, ASICs and P2X receptors signal a variety of major brain pathologies. Here we briefly outline the physiological roles of ASICs and P2X receptors, focusing on the brain pathologies involving these receptors.

Sa12b Peptide from Solitary Wasp Inhibits ASIC Currents in Rat Dorsal Root Ganglion Neurons

In this work, we evaluate the effect of two peptides Sa12b (EDVDHVFLRF) and Sh5b (DVDHVFLRF-NH₂) on Acid-Sensing Ion Channels (ASIC). These peptides were purified from the venom of solitary wasps Sphex argentatus argentatus and Isodontia harmandi, respectively. Voltage clamp recordings of ASIC currents were performed in whole cell configuration in primary culture of dorsal root ganglion (DRG) neurons from (P7-P10) CII Long-Evans rats. The peptides were applied by preincubation for 25 s (20 s in pH 7.4 solution and 5 s in pH 6.1 solution) or by co-application (5 s in pH 6.1 solution). Sa12b inhibits ASIC current with an IC₅₀ of 81 nM, in a concentration-dependent manner when preincubation application was used. While Sh5b did not show consistent results having both excitatory and inhibitory effects on the maximum ASIC currents, its complex effect suggests that it presents a selective action on some ASIC subunits. Despite the similarity in their sequences, the action of these peptides differs significantly. Sa12b is the first discovered wasp peptide with a significant ASIC inhibitory effect.

Analgesic Activity of Acid-Sensing Ion Channel 3 (ASIС3) Inhibitors: Sea Anemones Peptides Ugr9-1 and APETx2 versus Low Molecular Weight Compounds

Acid-sensing ion channel 3 (ASIC3) makes an important contribution to the development and maintenance of inflammatory and acid-induced pain. We compared different ASIC3 inhibitors (peptides from sea anemones (APETx2 and Ugr9-1) and nonpeptide molecules (sevanol and diclofenac)) in anti-inflammatory action and analgesic effects. All tested compounds had distinct effects on pH-induced ASIC3 current. APETx2 inhibited only transient current, whereas Ugr9-1 and sevanol decreased transient and sustained components of the current. The effect on mice was evaluated after administering an intramuscular injection in the acetic acid writhing pain model and the complete Freund's adjuvant-induced thermal hyperalgesia/inflammation test. The bell-shaped dependence of the analgesic effect was observed for APETx2 in the acetic acid-induced writhing test, as well as for sevanol and peptide Ugr9-1 in the thermal hyperalgesia test. This dependence could be evidence of the nonspecific action of compounds in high doses. Compounds reducing both components of ASIC3 current produced more significant pain relief than APETx2, which is an effective inhibitor of a transient current only. Therefore, the comparison of the efficacy of ASIC3 inhibitors revealed the importance of ASIC3-sustained currents' inhibition for promotion of acidosis-related pain relief.

Root bark of Discaria americana attenuates pain: A pharmacological evidence of interaction with opioidergic system and TRP/ASIC channels

ETHNOPHARMACOLOGICAL RELEVANCE

Discaria americana (Rhamnaceae) root bark infusion have been used in traditional medicine as antipyretic, tonic, ameliorative of stomach and skin diseases and diabetes. This study was designed to investigate whether the methanolic extract of the root bark of Discaria americana (MEDa) exhibits antinociceptive effects in mice. Furthermore, it was investigated the involvement of the opioidergic system in MEDa mechanism of action as well the interactions with TRP/ASIC channels in its effect.

MATERIALS AND METHODS

The antinociceptive effect of intra-gastric gavage (i.g.) of MEDa (0.3-300 mg/kg) was evaluated in mice subjected to acute chemical (acetic-acid, formalin, glutamate, capsaicin, cinnamaldehyde, and acidified saline) or thermal (hot plate) tests of pain. The involvement of opioid system was evaluated in the formalin test. A nonspecific effect of MEDa was observed by measuring locomotor activity and exploratory behavior in open field test.

RESULTS

MEDa significantly reduced the number of writhing induced by acetic acid and inhibited the nociception in the two phases of formalin. These effects were inhibited by pretreatment with naloxone. The nociception induced by hot plate and intraplantar injection of glutamate, capsaicin, cinnamaldehyde and acidified saline were significantly inhibited by MEDa. Only the dose of 300 mg/kg altered the locomotor activity.

CONCLUSIONS

Our results demonstrated, for the first time, that the methanolic extract of the root bark of Discaria americana presents antinociceptive effect in chemical and thermal stimuli and its analgesic properties can be due activation of the opioidergic system. These results support the use of Discaria americana in traditional medicine and demonstrate that this plant presents a therapeutic potential for the development of phytomedicines with antinociceptive profile.

Pharmacological evidence that DAPI inhibits NHE2 in Fundulus heteroclitus acclimated to freshwater

Ionoregulation in the euryhaline killifish Fundulus heteroclitus has been intensively studied over the last two decades using a variety of techniques. However, there has been limited use of pharmacological inhibitors to identify proteins involved in ion transport for this species. In this study, we used a range of pharmacological inhibitors (EIPA, DAPI, ethoxzolamide, bumetanide, bafilomycin, phenamil, hydrochlorothiazide) to investigate the proteins involved in Na⁺ transport in freshwater (1 mM Na⁺) acclimated F. heteroclitus. Our results indicate that Na⁺ uptake under these conditions is sensitive to both EIPA (NHE-specific inhibitor) and DAPI (putative ASIC-specific inhibitor), but not to any of the other inhibitors. Results for EIPA are consistent with previous studies indicating F. heteroclitus relies solely on NHE2 for Na⁺ transport across the apical membrane of ionocytes. In contrast, results for DAPI are surprising given previous studies that have indicated the H⁺-ATPase is basolaterally located in F. heteroclitus and so cannot contribute to Na⁺ uptake via ASIC. The lack of bafilomycin sensitivity in the current study is consistent with a basolaterally located H⁺-ATPase. This suggests that DAPI is not an ASIC-specific inhibitor as has been previously assumed, and that it may also inhibit NHE2. Finally, we did not observe Na⁺ uptake to be sensitive to ethoxzolamide, suggesting that carbonic anhydrase may not be involved in generating the H⁺ needed to maintain NHE activity in freshwater as has been previously proposed.

Selection of an ASIC1a-blocking combinatorial antibody that protects cells from ischemic death

Acid-sensing ion channels (ASICs) have emerged as important, albeit challenging therapeutic targets for pain, stroke, etc. One approach to developing therapeutic agents could involve the generation of functional antibodies against these channels. To select such antibodies, we used channels assembled in nanodiscs, such that the target ASIC1a has a configuration as close as possible to its natural state in the plasma membrane. This methodology allowed selection of functional antibodies that inhibit acid-induced opening of the channel in a dose-dependent way. In addition to regulation of pH, these antibodies block the transport of cations, including calcium, thereby preventing acid-induced cell death in vitro and in vivo. As proof of concept for the use of these antibodies to modulate ion channels in vivo, we showed that they potently protect brain cells from death after an ischemic stroke. Thus, the methodology described here should be general, thereby allowing selection of antibodies to other important ASICs, such as those involved in pain, neurodegeneration, and other conditions.

[The Biological Activity of the Sevanol and Its Analogues]

Previously, from the plant Thymus armeniacus a new lignan sevanol was isolated, it's structure was elucidated and was shown that it effectively inhibits the acid-sensing channel ASIC3 and also exhibits a pronounced analgesic and anti-inflammatory effect. In this work biological activity of the sevanol analog obtained by chemical synthesis from simple precursors, the stereoisomer of sevanol and a precursor molecule represents a half of sevanol was measured in electrophysiological experiments on human ASIC3 channels expressed in Xenopus laevis oocytes. Measured inhibitory activity of a synthetic analogue coincided with the activity ofthe natural molecule. Stereoisomer showed inhibitory activity drop by about a third part, and the precursor molecule showed much less significant activity. In result the significance of functional groups and a spatial configuration of sevanol in order to biological activity was shown that is important to take into account for the optimal synthesis design as well as for new drugs development on its base.

Amiloride suppresses pilocarpine-induced seizures via ASICs other than NHE in rats

BACKGROUND AND PURPOSE

Although recent studies have indicated that acid-sensing ion channels (ASICs) may play an important role in suppressing status epilepticus (SE) in rats, the precise mechanism is unclear. We attempted to investigate the antiepileptic effect of amiloride in SE rats and its mechanism.

METHODS

Rats with seizures induced by Li-pilocarpine were randomly divided into four groups, phosphate buffer saline (PBS) group, amiloride group, levetiracetam group and acidic liquid group, respectively. The electroencephalogram (EEG) of each group was recorded. Then rats treated with different drugs (2 h after amiloride or PBS injection or 1 h after PBS injection) and a normal control group was selected for reverse transcription-polymerase chain reaction (RT-PCR). The expression of ASIC1a, ASIC3 and sodium-hydrogen exchanger (NHE) in each group was detected.

RESULTS

Amiloride reduced the frequency of discharge in 60~90 min after injection significantly. In acidic liquid group, the epileptic discharge was increased in 0~30 min. Moreover, the expression of ASIC1a, ASIC3 and NHE was obviously increased in the SE groups. Compared with SE groups, the expression of ASIC1a and ASIC3 mRNA in amiloride group decreased significantly. While NHE mRNA expression in the SE groups showed no significant difference.

CONCLUSION

Amiloride inhibited pilocarpine-induced SE and the anti-epileptic mechanism was associated with deactivation of the ASIC1a and ASIC3 instead of NHE in rats.

ASICs as therapeutic targets for migraine

Migraine is the most common neurological disorder and one of the most common chronic pain conditions. Despite its prevalence, the pathophysiology leading to migraine is poorly understood and the identification of new therapeutic targets has been slow. Several processes are currently thought to contribute to migraine including altered activity in the hypothalamus, cortical-spreading depression (CSD), and afferent sensory input from the cranial meninges. Decreased extracellular pH and subsequent activation of acid-sensing ion channels (ASICs) may contribute to each of these processes and may thus play a role in migraine pathophysiology. Although few studies have directly examined a role of ASICs in migraine, studies directly examining a connection have generated promising results including efficacy of ASIC blockers in both preclinical migraine models and in human migraine patients. The purpose of this review is to discuss the pathophysiology thought to contribute to migraine and findings that implicate decreased pH and/or ASICs in these events, as well as propose issues to be resolved in future studies of ASICs and migraine. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

Effect of acute and prolonged alcohol administration on Mg(2+) homeostasis in cardiac cells

Alcoholic cardiomyopathy represents a major clinical complication in chronic alcoholics. Previous studies from our laboratory indicate that acute and chronic exposure of liver cells to ethanol results in a major loss of cellular Mg(2+) as a result of alcohol oxidation. We investigated whether exposure to ethanol induces a similar Mg(2+) loss in cardiac cells. The results indicate that chronic exposure to a 6% ethanol-containing diet depleted cardiac myocytes of >25% of their cellular Mg(2+) content. Acute ethanol exposure, instead, induced a time- and dose-dependent manner of Mg(2+) extrusion from perfused hearts and collagenase-dispersed cardiac ventricular myocytes. Pretreatment with chlormethiazole prevented ethanol-induced Mg(2+) loss to a large extent, suggesting a role of ethanol oxidation via cyP4502E1 in the process. Magnesium extrusion across the sarcolemma occurred via the amiloride-inhibited Na(+)/Mg(2+) exchanger. Taken together, our data indicate that Mg(2+) extrusion also occurs in cardiac cells exposed to ethanol as a result of alcohol metabolism by cyP4502E1. The extrusion, which is mediated by the Na(+)/Mg(2+) exchanger, only occurs at doses of ethanol ≥0.1%, and depends on ethanol-induced decline in cellular ATP. The significance of Mg(2+) extrusion for the onset of alcoholic cardiomyopathy remains to be elucidated.

Expression and activity of acid-sensing ion channels in the mouse anterior pituitary

Acid sensing ion channels (ASICs) are proton-gated cation channels that are expressed in the nervous system and play an important role in fear learning and memory. The function of ASICs in the pituitary, an endocrine gland that contributes to emotions, is unknown. We sought to investigate which ASIC subunits were present in the pituitary and found mRNA expression for all ASIC isoforms, including ASIC1a, ASIC1b, ASIC2a, ASIC2b, ASIC3 and ASIC4. We also observed acid-evoked ASIC-like currents in isolated anterior pituitary cells that were absent in mice lacking ASIC1a. The biophysical properties and the responses to PcTx1, amiloride, Ca2+ and Zn2+ suggested that ASIC currents were mediated predominantly by heteromultimeric channels that contained ASIC1a and ASIC2a or ASIC2b. ASIC currents were also sensitive to FMRFamide (Phe-Met-Arg-Phe amide), suggesting that FMRFamide-like compounds might endogenously regulate pituitary ASICs. To determine whether ASICs might regulate pituitary cell function, we applied low pH and found that it increased the intracellular Ca2+ concentration. These data suggest that ASIC channels are present and functionally active in anterior pituitary cells and may therefore influence their function.

Differences in acid-induced currents between oxytocin-mRFP1 and vasopressin-eGFP neurons isolated from the supraoptic and paraventricular nuclei of transgenic rats

The hypothalamic supraoptic nucleus (SON) and paraventricular nucleus (PVN) consists of two types of magnocellular neurosecretory cells, oxytocin (OXT) and arginine vasopressin (AVP). We generated and characterized rats that express an OXT-monomeric red fluorescent protein 1 (mRFP1) and an AVP-enhanced green fluorescent protein (eGFP) fusion transgene. These transgenic rats enable the visualization of OXT or AVP neurons. Taking advantage of this, we examined the differences between OXT-mRFP1 neurons and AVP-eGFP neurons in response to acid. Acid-sensing ion channels (ASICs) are neuronal voltage-insensitive cationic channels that are activated by extracellular acidification. Although functional ASICs have been identified in AVP neurons, differences in acid-induced currents between OXT and AVP neurons in SON have not been reported. In the present study, we used the whole-cell patch-clamp technique to investigate differences between OXT-mRFP1 neurons and AVP-eGFP neurons reaction to acid in SON and PVN. In voltage clamp mode, lowering extracellular pH evoked inward currents in both OXT-mRFP1 neurons and AVP-eGFP neurons. In our findings, the acid-induced currents in the OXT-mRFP1 neurons were significantly smaller than those in the AVP-eGFP neurons. These acid-induced currents were inhibited by amiloride, a known blocker of ASICs. Further, to compare the response to acid between OXT-mRFP1 and AVP-eGFP neurons in the same transgenic rat, we used a double transgenic rat by mating an OXT-mRFP1 transgenic rat with an AVP-eGFP transgenic rat. The acid-induced currents of OXT-mRFP1 neurons were significantly smaller than those of AVP-eGFP neurons from the double transgenic rats. These currents were almost completely inhibited by amiloride. The difference of acid-sensitivity between OXT and AVP neurons might contribute to maintaining systematic order in hypothalamic function.

Amiloride-sensitive sodium currents in fungiform taste cells of rats chronically exposed to nicotine

Many studies have demonstrated that chronic exposure to nicotine, one of the main components of tobacco smoke, has profound effects on the functionality of the mammalian taste system. However, the mechanisms underlying nicotine action are poorly understood. In particular no information is available on the chronic effect of nicotine on the functioning of taste cells, the peripheral detectors which transduce food chemicals into electrical signals to the brain. To address this issue, I studied the membrane properties of rat fungiform taste cells and evaluated the effect of long-term exposure to nicotine on the amiloride-sensitive sodium currents (ASSCs). These currents are mediated by the epithelial sodium channels (ENaC) thought to be important, at least in part, in the transduction of salty stimuli. Patch-clamp recording data indicated that ASSCs in taste cells from rats chronically treated with nicotine had a reduced amplitude compared to controls. The pharmacological and biophysical analysis of ASSCs revealed that amplitude reduction was not dependent on changes in amiloride sensitivity or channel ionic permeability, but likely derived from a decrease in the activity of ENaCs. Since these channels are considered to be sodium receptors in taste cells, my results suggest that chronic exposure to nicotine hampers the capability of these cells to respond to sodium ions. This might represent a possible cellular mechanism underlying the reduced taste sensitivity to salt typically found in smokers.

Hydrogen sulfide is involved in maintaining ion homeostasis via regulating plasma membrane Na+/H+ antiporter system in the hydrogen peroxide-dependent manner in salt-stress Arabidopsis thaliana root

Hydrogen sulfide (H2S) and hydrogen peroxide (H2O2) function as the signaling molecules in plants responding to salt stresses. The present study presents a signaling network involving H2S and H2O2 in salt resistance pathway of the Arabidopsis root. Arabidopsis roots were sensitive to 100 mM NaCl treatment, which displayed a great increase in electrolyte leakage (EL) and Na(+)/K(+) ratio under salt stress. The treatment of H2S donors sodium hydrosulfide (NaHS) enhanced the salt tolerance by maintaining a lower Na(+)/K(+) ratio. In addition, the inhibition of root growth under salt stress was removed by H2S. Further studies indicated that H2O2 was involved in H2S-induced salt tolerance pathway. H2S induced the production of the endogenous H2O2 via regulating the activities of glucose-6-phosphate dehydrogenase (G6PDH) and plasma membrane (PM) NADPH oxidase, with the treatment with dimethylthiourea (DMTU, an ROS scavenger), diphenylene iodonium (DPI, a PM NADPH oxidase inhibitor), or glycerol (G6PDH inhibitor) removing the effect of H2S. Treatment with amiloride (an inhibitor of PM Na(+)/H(+) antiporter) and vanadate (an inhibitor of PM H(+)-ATPase) also inhibited the activity of H2S on Na(+)/K(+) ratio. Through an analysis of quantitative real-time polymerase chain reaction and Western blot, we found that H2S promoted the genes expression and the phosphorylation level of PM H(+)-ATPase and Na(+)/H(+) antiporter protein level. However, when the endogenous H2O2 level was inhibited by DPI or DMTU, the effect of H2S on the PM Na(+)/H(+) antiporter system was removed. Taken together, H2S maintains ion homeostasis in the H2O2-dependent manner in salt-stress Arabidopsis root.

[Acid-Sensing Ion Channels (ASICs) in pain]

The discovery of new drug targets represents a real opportunity for developing fresh strategies against pain. Ion channels are interesting targets because they are directly involved in the detection and the transmission of noxious stimuli by sensory fibres of the peripheral nervous system and by neurons of the spinal cord. Acid-Sensing Ion Channels (ASICs) have emerged as important players in the pain pathway. They are neuronal, voltage-independent depolarizing sodium channels activated by extracellular protons. The ASIC family comprises several subunits that need to associate into homo- or hetero-trimers to form a functional channel. The ASIC1 and ASIC3 isoforms are particularly important in sensory neurons, whereas ASIC1a, alone or in association with ASIC2, is essential in the central nervous system. The potent analgesic effects associated with their inhibition in animals (which can be comparable to those of morphine) and data suggesting a role in human pain illustrate the therapeutic potential of these channels.

Gastrodin inhibits the activity of acid-sensing ion channels in rat primary sensory neurons

Acid-sensing ion channels (ASICs), a family of proton-gated cation channels, are believed to mediate pain caused by extracellular acidification. Gastrodin is a main bioactive constituent of the traditional herbal Gastrodia elata Blume, which has been widely used in Oriental countries for centuries. As an analgesic, gastrodin has been used clinically to treat pain such as migraine and headache. However, the mechanisms underlying analgesic action of gastrodin are still poorly understood. Here, we have found that gastrodin inhibited the activity of native ASICs in rat dorsal root ganglion (DRG) neurons. Gastrodin dose-dependently inhibited proton-gated currents mediated by ASICs. Gastrodin shifted the proton concentration-response curve downwards, with a decrease of 36.92 ± 6.23% in the maximum current response but with no significant change in the pH0.5 value. Moreover, gastrodin altered acid-evoked membrane excitability of rat DRG neurons and caused a significant decrease in the amplitude of the depolarization and the number of action potentials induced by acid stimuli. Finally, peripheral applied gastrodin relieved pain evoked by intraplantar injection of acetic acid in rats. Our results indicate that gastrodin can inhibit the activity of ASICs in the primary sensory neurons, which provided a novel mechanism underlying analgesic action of gastrodin.

Na+/H+ and Na+/NH+4 activities of zebrafish NHE3b expressed in Xenopus oocytes

Zebrafish Na(+)/H(+) exchanger 3b (zNHE3b) is highly expressed in the apical membrane of ionocytes where Na(+) is absorbed from ion-poor fresh water against a concentration gradient. Much in vivo data indicated that zNHE3b is involved in Na(+) absorption but not leakage. However, zNHE3b-mediated Na(+) absorption has not been thermodynamically explained, and zNHE3b activity has not been measured. To address this issue, we overexpressed zNHE3b in Xenopus oocytes and characterized its activity by electrophysiology. Exposure of zNHE3b oocytes to Na(+)-free media resulted in significant decrease in intracellular pH (pH(i)) and intracellular Na(+) activity (aNa(i)). aNa(i) increased significantly when the cytoplasm was acidified by media containing CO₂-HCO₃(-) or butyrate. Activity of zNHE3b was inhibited by amiloride or 5-ethylisopropyl amiloride (EIPA). Although the activity was accompanied by a large hyperpolarization of ∼50 mV, voltage-clamp experiments showed that Na(+)/H(+) exchange activity of zNHE3b is electroneutral. Exposure of zNHE3b oocytes to medium containing NH₃/NH₄(+) resulted in significant decreases in pH(i) and aNa(i) and significant increase in intracellular NH₄(+) activity, indicating that zNHE3b mediates the Na(+)/NH₄(+) exchange. In low-Na(+) (0.5 mM) media, zNHE3b oocytes maintained aNa(i) of 1.3 mM, and Na(+)-influx was observed when pHi was decreased by media containing CO₂-HCO₃(-) or butyrate. These results provide thermodynamic evidence that zNHE3b mediates Na(+) absorption from ion-poor fresh water by its Na(+)/H(+) and Na(+)/NH₄(+) exchange activities.

Emerging targets in migraine

Migraine is a common and highly disabling neurological disorder. Despite the complexity of its pathophysiology, substantial advances have been achieved over the past 20 years in its understanding, as well as the development of pharmacological treatment options. The development of serotonin 5-HT(1B/1D) receptor agonists ("triptans") substantially improved the acute treatment of migraine attacks. However, many migraineurs do not respond satisfactorily to triptans and cardiovascular co-morbidities limit their use in a significant number of patients. As migraine is increasingly considered to be a disorder of the brain, and preclinical and clinical data indicate that the observed vasodilation is merely an epiphenomenon, research has recently focused on the development of neurally acting compounds that lack vasoconstrictor properties. This review highlights the most important pharmacological targets for which compounds have been developed that are highly likely to enter or have already advanced into clinical trials for the acute and preventive treatment of migraine. In this context, preclinical and clinical data on compounds acting on calcitonin gene-related peptide or its receptor, the 5-HT(1F) receptor, nitric oxide synthase, and acid-sensing ion channel blockers are discussed.

Acid sensing ion channel 1 in lateral hypothalamus contributes to breathing control

Acid-sensing ion channels (ASICs) are present in neurons and may contribute to chemoreception. Among six subunits of ASICs, ASIC1 is mainly expressed in the central nervous system. Recently, multiple sites in the brain including the lateral hypothalamus (LH) have been found to be sensitive to extracellular acidification. Since LH contains orexin neurons and innervates the medulla respiratory center, we hypothesize that ASIC1 is expressed on the orexin neuron and contributes to acid-induced increase in respiratory drive. To test this hypothesis, we used double immunofluorescence to determine whether ASIC1 is expressed on orexin neurons in the LH, and assessed integrated phrenic nerve discharge (iPND) in intact rats in response to acidification of the LH. We found that ASIC1 was co-localized with orexinA in the LH. Microinjection of acidified artificial cerebrospinal fluid increased the amplitude of iPND by 70% (pH 7.4 v.s. pH 6.5:1.05±0.12 v.s. 1.70±0.10, n = 6, P<0.001) and increased the respiratory drive (peak amplitude of iPND/inspiratory time, PA/Ti) by 40% (1.10±0.23 v.s. 1.50±0.38, P<0.05). This stimulatory effect was abolished by blocking ASIC1 with a nonselective inhibitor (amiloride 10 mM), a selective inhibitor (PcTX1, 10 nM) or by damaging orexin neurons in the LH. Current results support our hypothesis that the orexin neuron in the LH can exert an excitation on respiration via ASIC1 during local acidosis. Since central acidification is involved in breathing dysfunction in a variety of pulmonary diseases, understanding its underlying mechanism may improve patient management.

Cyclisation increases the stability of the sea anemone peptide APETx2 but decreases its activity at acid-sensing ion channel 3

APETx2 is a peptide isolated from the sea anemone Anthopleura elegantissima. It is the most potent and selective inhibitor of acid-sensing ion channel 3 (ASIC3) and it is currently in preclinical studies as a novel analgesic for the treatment of chronic inflammatory pain. As a peptide it faces many challenges in the drug development process, including the potential lack of stability often associated with therapeutic peptides. In this study we determined the susceptibility of wild-type APETx2 to trypsin and pepsin and tested the applicability of backbone cyclisation as a strategy to improve its resistance to enzymatic degradation. Cyclisation with either a six-, seven- or eight-residue linker vastly improved the protease resistance of APETx2 but substantially decreased its potency against ASIC3. This suggests that either the N- or C-terminus of APETx2 is involved in its interaction with the channel, which we confirmed by making N- and C-terminal truncations. Truncation of either terminus, but especially the N-terminus, has detrimental effects on the ability of APETx2 to inhibit ASIC3. The current work indicates that cyclisation is unlikely to be a suitable strategy for stabilising APETx2, unless linkers can be engineered that do not interfere with binding to ASIC3.

Functional expression in Escherichia coli of the disulfide-rich sea anemone peptide APETx2, a potent blocker of acid-sensing ion channel 3

Acid-sensing ion channels (ASICs) are proton-gated sodium channels present in the central and peripheral nervous system of chordates. ASIC3 is highly expressed in sensory neurons and plays an important role in inflammatory and ischemic pain. Thus, specific inhibitors of ASIC3 have the potential to be developed as novel analgesics. APETx2, isolated from the sea anemone Anthopleura elegantissima, is the most potent and selective inhibitor of ASIC3-containing channels. However, the mechanism of action of APETx2 and the molecular basis for its interaction with ASIC3 is not known. In order to assist in characterizing the ASIC3-APETx2 interaction, we developed an efficient and cost-effective Escherichia coli periplasmic expression system for the production of APETx2. NMR studies on uniformly (13)C/(15)N-labelled APETx2 produced in E. coli showed that the recombinant peptide adopts the native conformation. Recombinant APETx2 is equipotent with synthetic APETx2 at inhibiting ASIC3 channels expressed in Xenopus oocytes. Using this system we mutated Phe15 to Ala, which caused a profound loss of APETx2's activity on ASIC3. These findings suggest that this expression system can be used to produce mutant versions of APETx2 in order to facilitate structure-activity relationship studies.