ASIC3 and ASIC1 mediate FMRFamide-related peptide enhancement of H+-gated currents in cultured dorsal root ganglion neurons

Proton-sensing ion channels, GPCRs and calcium signaling regulated by them: implications for cancer

Extracellular acidification of tumors is common. Through proton-sensing ion channels or proton-sensing G protein-coupled receptors (GPCRs), tumor cells sense extracellular acidification to stimulate a variety of intracellular signaling pathways including the calcium signaling, which consequently exerts global impacts on tumor cells. Proton-sensing ion channels, and proton-sensing GPCRs have natural advantages as drug targets of anticancer therapy. However, they and the calcium signaling regulated by them attracted limited attention as potential targets of anticancer drugs. In the present review, we discuss the progress in studies on proton-sensing ion channels, and proton-sensing GPCRs, especially emphasizing the effects of calcium signaling activated by them on the characteristics of tumors, including proliferation, migration, invasion, metastasis, drug resistance, angiogenesis. In addition, we review the drugs targeting proton-sensing channels or GPCRs that are currently in clinical trials, as well as the relevant potential drugs for cancer treatments, and discuss their future prospects. The present review aims to elucidate the important role of proton-sensing ion channels, GPCRs and calcium signaling regulated by them in cancer initiation and development. This review will promote the development of drugs targeting proton-sensing channels or GPCRs for cancer treatments, effectively taking their unique advantage as anti-cancer drug targets.

The Impact of Plasma Membrane Ion Channels on Bone Remodeling in Response to Mechanical Stress, Oxidative Imbalance, and Acidosis

The extracellular milieu is a rich source of different stimuli and stressors. Some of them depend on the chemical–physical features of the matrix, while others may come from the ‘outer’ environment, as in the case of mechanical loading applied on the bones. In addition to these forces, a plethora of chemical signals drives cell physiology and fate, possibly leading to dysfunctions when the homeostasis is disrupted. This variety of stimuli triggers different responses among the tissues: bones represent a particular milieu in which a fragile balance between mechanical and metabolic demands should be tuned and maintained by the concerted activity of cell biomolecules located at the interface between external and internal environments. Plasma membrane ion channels can be viewed as multifunctional protein machines that act as rapid and selective dual-nature hubs, sensors, and transducers. Here we focus on some multisensory ion channels (belonging to Piezo, TRP, ASIC/EnaC, P2XR, Connexin, and Pannexin families) actually or potentially playing a significant role in bone adaptation to three main stressors, mechanical forces, oxidative stress, and acidosis, through their effects on bone cells including mesenchymal stem cells, osteoblasts, osteoclasts, and osteocytes. Ion channel-mediated bone remodeling occurs in physiological processes, aging, and human diseases such as osteoporosis, cancer, and traumatic events.

Evolution of the insect PPK gene family

Insect pickpocket (PPK) receptors mediate diverse functions, among them the detection of mechano- and chemo-sensory stimuli. Notwithstanding their relevance, studies on their evolution only focused on Drosophila. We have analyzed the genomes of 26 species of 8 orders including holometabolous and hemimetabolous insects (Blattodea, Orthoptera, Hemiptera, Phthiraptera, Hymenoptera, Lepidoptera, Coleoptera, and Diptera), to characterize the evolution of this gene family. PPKs were detected in all genomes analyzed, with 578 genes distributed in 7 subfamilies. According to our phylogeny ppk17 is the most divergent member, composing the new subfamily VII. PPKs evolved under a gene birth-and-death model that generated lineage-specific expansions usually located in clusters, while purifying selection affected several orthogroups. Subfamily V was the largest, including a mosquito-specific expansion that can be considered a new target for pest control. PPKs present a high gene turnover generating considerable variation. On one hand, Musca domestica (59), Aedes albopictus (51), Culex quinquefasciatus (48), and Blattella germanica (41) presented the largest PPK repertoires. On the other hand, Pediculus humanus (only ppk17), bees and ants (6-9) had the smallest PPK sets. A subset of prevalent PPKs was identified, indicating very conserved functions for these receptors. Finally, at least twenty percent of the sequences presented calmodulin-binding motifs, suggesting that these PPKs may amplify sensory responses similarly as proposed for D. melanogaster ppk25. Overall, this work characterized the evolutionary history of these receptors revealing relevant unknown gene sequence features and clade-specific expansions.

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.

Non-proton ligand-sensing domain of acid-sensing ion channel 3 is required for itch sensation

Itch, the unpleasant sensation that evokes a desire to scratch, accompanies numerous skin and nervous system disorders. However, the molecular mechanisms of itch are unclear. Acid-sensing ion channel 3 (ASIC3) is a sensor of acidic and primary inflammatory pain. The whole-cell patch clamp technique was used to determine the effect of chloroquine (CQ) on ASICs currents in primary sensory neurons or the Chinese hamster ovary cells transfected with rat ASIC1a or ASIC3. Site-directed mutagenesis of plasmid was performed. Scratching behavior was evaluated by measuring the number of bouts during 30 min after injection. CQ, an anti-malarial drug defined as a histamine-independent pruritogen, selectively enhanced the sustained phase of ASIC3 current in a concentration-dependent manner either in ASIC3-transfected Chinese hamster ovary cells or in primary cultured rat dorsal root ganglion neurons. Further studies revealed that the effect of CQ on ASIC3 channels depends on the newly identified non-proton ligand-sensing domain. Importantly, CQ-evoked scratching behavior was largely alleviated by APETx2, a selective ASIC3 channel blocker. Like CQ, other compounds such as amiloride, 2-guanidine-4-methylquinazoline and neuropeptide FF, which have been previously reported to be non-proton ligands that activate ASIC3, undoubtedly evoked the scratching response. In conclusion, ASIC3, a proton-gated ion channel critical for pain sensation, also functions as an essential component of itch transduction.

ASICs Mediate Pain and Inflammation in Musculoskeletal Diseases

Chronic musculoskeletal pain is debilitating and affects ∼ 20% of adults. Tissue acidosis is present in painful musculoskeletal diseases like rheumatoid arthritis. ASICs are located on skeletal muscle and joint nociceptors as well as on nonneuronal cells in the muscles and joints, where they mediate nociception. This review discusses the properties of different types of ASICs, factors affecting their pH sensitivity, and their role in musculoskeletal hyperalgesia and inflammation.

Acid-sensing ion channel (ASIC) 4 predominantly localizes to an early endosome-related organelle upon heterologous expression

Acid-sensing ion channels (ASICs) are voltage-independent proton-gated amiloride sensitive sodium channels, belonging to the DEG/ENaC gene family. Six different ASICs have been identified (ASIC1a, ASIC1b, ASIC2a, ASIC2b, ASIC3, ASIC4) that are activated by a drop in extracellular pH, either as homo- or heteromers. An exception is ASIC4, which is not activated by protons as a homomer and which does not contribute to functional heteromeric ASICs. Insensitivity of ASIC4 to protons and its comparatively low sequence identity to other ASICs (45%) raises the question whether ASIC4 may have different functions than other ASICs. In this study, we therefore investigated the subcellular localization of ASIC4 in heterologous cell lines, which revealed a surprising accumulation of the channel in early endosome-related vacuoles. Moreover, we identified an unique amino-terminal motif as important for forward-trafficking from the ER/Golgi to the early endosome-related compartment. Collectively, our results show that heterologously expressed ASIC4 predominantly resides in an intracellular endosomal compartment.

Peptide-gated ion channels and the simple nervous system of Hydra

Neurons either use electrical or chemical synapses to communicate with each other. Transmitters at chemical synapses are either small molecules or neuropeptides. After binding to their receptors, transmitters elicit postsynaptic potentials, which can either be fast and transient or slow and longer lasting, depending on the type of receptor. Fast transient potentials are mediated by ionotropic receptors and slow long-lasting potentials by metabotropic receptors. Transmitters and receptors are well studied for animals with a complex nervous system such as vertebrates and insects, but much less is known for animals with a simple nervous system like Cnidaria. As cnidarians arose early in animal evolution, nervous systems might have first evolved within this group and the study of neurotransmission in cnidarians might reveal an ancient mechanism of neuronal communication. The simple nervous system of the cnidarian Hydra extensively uses neuropeptides and, recently, we cloned and functionally characterized an ion channel that is directly activated by neuropeptides of the Hydra nervous system. These results demonstrate the existence of peptide-gated ion channels in Hydra, suggesting they mediate fast transmission in its nervous system. As related channels are also present in the genomes of the cnidarian Nematostella, of placozoans and of ctenophores, it should be considered that the early nervous systems of cnidarians and ctenophores have co-opted neuropeptides for fast transmission at chemical synapses.

Unique roles played by Acid-sensing ion channel 2

The discovery of Acid-sensing ion channels (ASICs) provided us the theoretical basis to understand the pathological acidic environment. They belong to the degenerin/epithelial Na+ channel family and function once extracellular pH decreases to a certain level, and this characteristic make them spotlights in the regulation or response of pH change. As a regulatory system, keeping the intra- and extra-balance seems to be significant for ASICs, in which ASIC2 plays an important role. We surprisingly noticed that ASIC2 owns some distinctive properties, including its inter-system regulation, specific distribution and transporting patterns, influence on cell migration and the unique role in mechanosensitivity. Therefore, to conclude the functions and characterisitics of ASIC2 indeed assist the understanding of interaction among ASICs subunits and the regulation from extracellular environment to ASICs.

ASICs and neuropeptides

The acid sensing ion channels (ASICs) are proton-gated cation channels expressed throughout the nervous system. ASICs are activated during acidic pH fluctuations, and recent work suggests that they are involved in excitatory synaptic transmission. ASICs can also induce neuronal degeneration and death during pathological extracellular acidosis caused by ischemia, autoimmune inflammation, and traumatic injury. Many endogenous neuromodulators target ASICs to affect their biophysical characteristics and contributions to neuronal activity. One of the most unconventional types of modulation occurs with the interaction of ASICs and neuropeptides. Collectively, FMRFamide-related peptides and dynorphins potentiate ASIC activity by decreasing the proton-sensitivity of steady state desensitization independent of G protein-coupled receptor activation. By decreasing the proton-sensitivity of steady state desensitization, the FMRFamide-related peptides and dynorphins permit ASICs to remain active at more acidic basal pH. Unlike the dynorphins, some FMRFamide-related peptides also potentiate ASIC activity by slowing inactivation and increasing the sustained current. Through mechanistic studies, the modulation of ASICs by FMRFamide-related peptides and dynorphins appears to be through distinct interactions with the extracellular domain of ASICs. Dynorphins are expressed throughout the nervous system and can increase neuronal death during prolonged extracellular acidosis, suggesting that the interaction between dynorphins and ASICs may have important consequences for the prevention of neurological injury. The overlap in expression of FMRFamide-related peptides with ASICs in the dorsal horn of the spinal cord suggests that their interaction may have important consequences for the treatment of pain during injury and inflammation. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

Biophysical properties of acid-sensing ion channels (ASICs)

Acid-sensing ion channels (ASICs) are ligand-gated ion channels that are exquisitely sensitive to extracellular protons and can sense transient as well as sustained acidification. In this review, we will discuss activation and desensitization of ASICs by protons. We show that a linear reaction scheme can reproduce the basic electrophysiological properties of ASICs, including steady-state desensitization. Moreover, we will discuss how a desensitizing receptor can sense sustained acidosis and what we know about the putative proton sensor. We will briefly discuss modulation of proton gating by neuropeptides and small positively charged ligands. Finally, we will review the pore properties of ASICs and their relation to the recently reported crystal structure of the open ASIC pore. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

The how and why of identifying the hair cell mechano-electrical transduction channel

Identification of the auditory hair cell mechano-electrical transduction (hcMET) channel has been a major focus in the hearing research field since the 1980s when direct mechanical gating of a transduction channel was proposed (Corey and Hudspeth J Neurosci 3:962-976, 1983). To this day, the molecular identity of this channel remains controversial. However, many of the hcMET channel's properties have been characterized, including pore properties, calcium-dependent ion permeability, rectification, and single channel conductance. At this point, elucidating the molecular identity of the hcMET channel will provide new tools for understanding the mechanotransduction process. This review discusses the significance of identifying the hcMET channel, the difficulties associated with that task, as well as the establishment of clear criteria for this identification. Finally, we discuss potential candidate channels in light of these criteria.

Conformational changes in the lower palm domain of ASIC1a contribute to desensitization and RFamide modulation

Acid-sensing ion channel 1a (ASIC1a) is a proton-gated cation channel that contributes to fear and pain as well as neuronal damage following persistent cerebral acidosis. Neuropeptides can affect acid-induced neuronal injury by altering ASIC1a inactivation and/or steady-state desensitization. Yet, exactly how ASIC1a inactivation and desensitization occur or are modulated by peptides is not completely understood. We found that regions of the extracellular palm domain and the β_11-12 linker are important for inactivation and steady-state desensitization of ASIC1a. The single amino acid substitutions L280C and L415C dramatically enhanced the rate of inactivation and altered the pH-dependence of steady-state desensitization. Further, the use of methanethiosulfonate (MTS) reagents suggests that the lower palm region (L280C) undergoes a conformational change when ASIC1a transitions from closed to desensitized. We determined that L280C also displays an altered response to the RFamide peptide, FRRFamide. Further, the presence of FRRFamide limited MTS modification of L280C. Together, these results indicate a potential role of the lower palm domain in peptide modulation and suggest RFamide-related peptides promote conformational changes within this region. These data provide empirical support for the idea that L280, and likely this region of the central vestibule, is intimately involved in channel inactivation and desensitization.

Serotonin facilitates peripheral pain sensitivity in a manner that depends on the nonproton ligand sensing domain of ASIC3 channel

Tissue acidosis and inflammatory mediators play critical roles in inflammatory pain. Extracellular acidosis activates acid-sensing ion channels (ASICs), which have emerged as key sensors for extracellular protons in the central and peripheral nervous systems and play key roles in pain sensation and transmission. Additionally, inflammatory mediators, such as serotonin (5-HT), are known to enhance pain sensation. However, functional interactions among protons, inflammatory mediators, and ASICs in pain sensation are poorly understood. In the present study, we show that 5-HT, a classical pro-inflammatory mediator, specifically enhances the proton-evoked sustained, but not transient, currents mediated by homomeric ASIC3 channels and heteromeric ASIC3/1a and ASIC3/1b channels. Unexpectedly, the effect of 5-HT on ASIC3 channels does not involve activation of 5-HT receptors, but is mediated via a functional interaction between 5-HT and ASIC3 channels. We further show that the effect of 5-HT on ASIC3 channels depends on the newly identified nonproton ligand sensing domain. Finally, coapplication of 5-HT and acid significantly increased pain-related behaviors as assayed by the paw-licking test in mice, which was largely attenuated in ASIC3 knock-out mice, and inhibited by the nonselective ASIC inhibitor amiloride. Together, these data identify ASIC3 channels as an unexpected molecular target for acute actions of 5-HT in inflammatory pain sensation and reveal an important role of ASIC3 channels in regulating inflammatory pain via coincident detection of extracellular protons and inflammatory mediators.

The genetic architecture of degenerin/epithelial sodium channels in Drosophila

Degenerin/epithelial sodium channels (DEG/ENaC) represent a large family of animal-specific membrane proteins. Although the physiological functions of most family members are not known, some have been shown to act as nonvoltage gated, amiloride-sensitive sodium channels. The DEG/ENaC family is exceptionally large in genomes of Drosophila species relative to vertebrates and other insects. To elucidate the evolutionary history of the DEG/ENaC family in Drosophila, we took advantage of the genomic and genetic information available for 12 Drosophila species that represent all the major species groups in the Drosophila clade. We have identified 31 family members (termed pickpocket genes) in Drosophila melanogaster, which can be divided into six subfamilies, which are represented in all 12 species. Structure prediction analyses suggested that some subunits evolved unique structural features in the large extracellular domain, possibly supporting mechanosensory functions. This finding is further supported by experimental data that show that both ppk1 and ppk26 are expressed in multidendritic neurons, which can sense mechanical nociceptive stimuli in larvae. We also identified representative genes from five of the six DEG/ENaC subfamilies in a mosquito genome, suggesting that the core DEG/ENaC subfamilies were already present early in the dipteran radiation. Spatial and temporal analyses of expression patterns of the various pickpocket genes indicated that paralogous genes often show very different expression patterns, possibly indicating that gene duplication events have led to new physiological or cellular functions rather than redundancy. In summary, our analyses support a rapid early diversification of the DEG/ENaC family in Diptera followed by physiological and/or cellular specialization. Some members of the family may have diversified to support the physiological functions of a yet unknown class of ligands.

Acid-sensing ion channels in pathological conditions

Acid-sensing ion channels (ASICs), a novel family of proton-gated amiloride-sensitive cation channels, are expressed primarily in neurons of peripheral sensory and central nervous systems. Recent studies have shown that activation of ASICs, particularly the ASIC1a channels, plays a critical role in neuronal injury associated with neurological disorders such as brain ischemia, multiple sclerosis, and spinal cord injury. In normal conditions in vitro, ASIC1a channels desensitize rapidly in the presence of a continuous acidosis or following a preexposure to minor pH drop, raising doubt for their contributions to the acidosis-mediated neuronal injury. It is now known that the properties of ASICs can be dramatically modulated by signaling molecules or biochemical changes associated with pathological conditions. Modulation of ASICs by these molecules can lead to dramatically enhanced and/or prolonged activities of these channels, thus promoting their pathological functions. Understanding of how ASICs behave in pathological conditions may help define new strategies for the treatment and/or prevention of neuronal injury associated with various neurological disorders.

Two mechanisms involved in trigeminal CGRP release: implications for migraine treatment

OBJECTIVE/BACKGROUND

The goal of this study was to better understand the cellular mechanisms involved in proton stimulation of calcitonin gene-related peptide (CGRP) secretion from cultured trigeminal neurons by investigating the effects of 2 antimigraine therapies, onabotulinumtoxinA and rizatriptan. Stimulated CGRP release from peripheral and central terminating processes of trigeminal ganglia neurons is implicated in migraine pathology by promoting inflammation and nociception. Based on models of migraine pathology, several inflammatory molecules including protons are thought to facilitate sensitization and activation of trigeminal nociceptive neurons and stimulate CGRP secretion. Despite the reported efficacy of triptans and onabotulinumtoxinA to treat acute and chronic migraine, respectively, a substantial number of migraineurs do not get adequate relief with these therapies. A possible explanation is that triptans and onabotulinumtoxinA are not able to block proton-mediated CGRP secretion.

METHODS

CGRP secretion from cultured primary trigeminal ganglia neurons was quantitated by radioimmunoassay while intracellular calcium and sodium levels were measured in neurons via live cell imaging using Fura-2 AM and SBFI AM, respectively. The expression of acid-sensing ion channel 3 (ASIC3) was determined by immunocytochemistry and Western blot analysis. In addition, the involvement of ASICs in mediating proton stimulation of CGRP was investigated using the potent and selective ASIC3 inhibitor APETx2.

RESULTS

While KCl caused a significant increase in CGRP secretion that was significantly repressed by treatment with ethylene glycol tetraacetic acid (EGTA), onabotulinumtoxinA, and rizatriptan, the stimulatory effect of protons (pH 5.5) was not suppressed by EGTA, onabotulinumtoxinA, or rizatriptan. In addition, while KCl caused a transient increase in intracellular calcium levels that was blocked by EGTA, no appreciable change in calcium levels was observed with proton treatment. However, protons did significantly increase the intracellular level of sodium ions. Under our culture conditions, ASIC3 was shown to be expressed in most trigeminal ganglion neurons. Importantly, proton stimulation of CGRP secretion was repressed by pretreatment with the ASIC3 inhibitor APETx2, but not the transient receptor potential vanilloid-1 antagonist capsazepine.

CONCLUSIONS

Our findings provide evidence that proton regulated release of CGRP from trigeminal neurons utilizes a different mechanism than the calcium and synaptosomal-associated protein 25-dependent pathways that are inhibited by the antimigraine therapies, rizatriptan and onabotulinumtoxinA.

Structure and activity of the acid-sensing ion channels

The acid-sensing ion channels (ASICs) are a family of proton-sensing channels expressed throughout the nervous system. Their activity is linked to a variety of complex behaviors including fear, anxiety, pain, depression, learning, and memory. ASICs have also been implicated in neuronal degeneration accompanying ischemia and multiple sclerosis. As a whole, ASICs represent novel therapeutic targets for several clinically important disorders. An understanding of the correlation between ASIC structure and function will help to elucidate their mechanism of action and identify potential therapeutics that specifically target these ion channels. Despite the seemingly simple nature of proton binding, multiple studies have shown that proton-dependent gating of ASICs is quite complex, leading to activation and desensitization through distinct structural components. This review will focus on the structural aspects of ASIC gating in response to both protons and the newly discovered activators GMQ and MitTx. ASIC modulatory compounds and their action on proton-dependent gating will also be discussed. This review is dedicated to the memory of Dale Benos, who made a substantial contribution to our understanding of ASIC activity.

FMRFamide-related peptides: anti-opiate transmitters acting in apoptosis

Members of the FMRFamide-related peptide (FaRP) family are neurotransmitters, hormone-like substances and tumor suppressor peptides. In mammals, FaRPs are considered as anti-opiate peptides due to their ability to inhibit opioid signaling. Some FaRPs are asserted to attenuate opiate tolerance. A recently developed chimeric FaRP (Met-enkephalin-FMRFa) mimics the analgesic effects of opiates without the development of opiate-dependence, displaying a future therapeutical potential in pain reduction. In this review we support the notion, that opiates and representative members of the FaRP family show overlapping effects on apoptosis. Binding of FaRPs to opioid receptors or to their own receptors (G-protein linked membrane receptors and acid-sensing ion channels) evokes or suppresses cell death, in a cell- and receptor-type manner. With the dramatically increasing incidence of opiate abuse and addiction, understanding of opioid-induced cell death, and in this context FaRPs will deserve growing attention.

Direct activation of guinea pig vagal afferent neurons by FMRFamide

Vagus nerve comprises two distinct kinds of nerves, nodose and jugular ganglionic nerves. We tested pharmacological difference between two vagal nerves in the responsiveness to FMRFamide. The response probability to FMRFamide was significantly higher in nodose than jugular nerves in intracellular calcium measurement. Nodose nerves also depolarized membrane potential to FMRFamide more than jugular nerves did, in patch-clamp recording, although the probability of action potential discharge was same in both nerves. The inward current induced by FMRFamide was characterized as mixed cations. These results suggest that FMRFamide may act as an activator and modulator of vagal sensory nerves for treating symptoms in visceral diseases.

ASIC3 channels in multimodal sensory perception

Acid-sensing ion channels (ASICs), which are members of the sodium-selective cation channels belonging to the epithelial sodium channel/degenerin (ENaC/DEG) family, act as membrane-bound receptors for extracellular protons as well as nonproton ligands. At least five ASIC subunits have been identified in mammalian neurons, which form both homotrimeric and heterotrimeric channels. The highly proton sensitive ASIC3 channels are predominantly distributed in peripheral sensory neurons, correlating with their roles in multimodal sensory perception, including nociception, mechanosensation, and chemosensation. Different from other ASIC subunit composing ion channels, ASIC3 channels can mediate a sustained window current in response to mild extracellular acidosis (pH 7.3-6.7), which often occurs accompanied by many sensory stimuli. Furthermore, recent evidence indicates that the sustained component of ASIC3 currents can be enhanced by nonproton ligands including the endogenous metabolite agmatine. In this review, we first summarize the growing body of evidence for the involvement of ASIC3 channels in multimodal sensory perception and then discuss the potential mechanisms underlying ASIC3 activation and mediation of sensory perception, with a special emphasis on its role in nociception. We conclude that ASIC3 activation and modulation by diverse sensory stimuli represent a new avenue for understanding the role of ASIC3 channels in sensory perception. Furthermore, the emerging implications of ASIC3 channels in multiple sensory dysfunctions including nociception allow the development of new pharmacotherapy.

Sensory functions for degenerin/epithelial sodium channels (DEG/ENaC)

All animals use a sophisticated array of receptor proteins to sense their external and internal environments. Major advances have been made in recent years in understanding the molecular and genetic bases for sensory transduction in diverse modalities, indicating that both metabotropic and ionotropic pathways are important in sensory functions. Here, I review the historical background and recent advances in understanding the roles of a relatively newly discovered family of receptors, the degenerin/epithelial sodium channels (DEG/ENaC). These animal-specific cation channels show a remarkable sequence and functional diversity in different species and seem to exert their functions in diverse sensory modalities. Functions for DEG/ENaC channels have been implicated in mechanosensation as well as chemosensory transduction pathways. In spite of overall sequence diversity, all family members share a unique protein topology that includes just two transmembrane domains and an unusually large and highly structured extracellular domain, that seem to be essential for both their mechanical and chemical sensory functions. This review will discuss many of the recent discoveries and controversies associated with sensory function of DEG/ENaC channels in both vertebrate and invertebrate model systems, covering the role of family members in taste, mechanosensation, and pain.

ASIC3 channels integrate agmatine and multiple inflammatory signals through the nonproton ligand sensing domain

Background

Acid-sensing ion channels (ASICs) have long been known to sense extracellular protons and contribute to sensory perception. Peripheral ASIC3 channels represent natural sensors of acidic and inflammatory pain. We recently reported the use of a synthetic compound, 2-guanidine-4-methylquinazoline (GMQ), to identify a novel nonproton sensing domain in the ASIC3 channel, and proposed that, based on its structural similarity with GMQ, the arginine metabolite agmatine (AGM) may be an endogenous nonproton ligand for ASIC3 channels.

Results

Here, we present further evidence for the physiological correlation between AGM and ASIC3. Among arginine metabolites, only AGM and its analog arcaine (ARC) activated ASIC3 channels at neutral pH in a sustained manner similar to GMQ. In addition to the homomeric ASIC3 channels, AGM also activated heteromeric ASIC3 plus ASIC1b channels, extending its potential physiological relevance. Importantly, the process of activation by AGM was highly sensitive to mild acidosis, hyperosmolarity, arachidonic acid (AA), lactic acid and reduced extracellular Ca²⁺. AGM-induced ASIC3 channel activation was not through the chelation of extracellular Ca²⁺ as occurs with increased lactate, but rather through a direct interaction with the newly identified nonproton ligand sensing domain. Finally, AGM cooperated with the multiple inflammatory signals to cause pain-related behaviors in an ASIC3-dependent manner.

Conclusions

Nonproton ligand sensing domain might represent a novel mechanism for activation or sensitization of ASIC3 channels underlying inflammatory pain-sensing under in vivo conditions.

Acid-sensing ion channels (ASICs): pharmacology and implication in pain

Tissue acidosis is a common feature of many painful conditions. Protons are indeed among the first factors released by injured tissues, inducing a local pH fall that depolarizes peripheral free terminals of nociceptors and leads to pain. ASICs are excitatory cation channels directly gated by extracellular protons that are expressed in the nervous system. In sensory neurons, they act as "chemo-electrical" transducers and are involved in somatic and visceral nociception. Two highly specific inhibitory peptides isolated from animal venoms have considerably helped in the understanding of the physiological roles of these channels in pain. At the peripheral level, ASIC3 is important for inflammatory pain. Its expression and its activity are potentiated by several pain mediators present in the "inflammatory soup" that sensitize nociceptors. ASICs have also been involved in some aspects of mechanosensation and mechanonociception, notably in the gastrointestinal tract, but the underlying mechanisms remain to be determined. At the central level, ASIC1a is largely expressed in spinal cord neurons where it has been proposed to participate in the processing of noxious stimuli and in central sensitization. Blocking ASIC1a in the spinal cord also produces a potent analgesia in a broad range of pain conditions through activation of the opiate system. Targeting ASIC channels at different levels of the nervous system could therefore be an interesting strategy for the relief of pain.

Acid-Sensing Ion Channels and Pain

Pathophysiological conditions such as inflammation, ischemia, infection and tissue injury can all evoke pain, and each is accompanied by local acidosis. Acid sensing ion channels (ASICs) are proton-gated cation channels expressed in both central and peripheral nervous systems. Increasing evidence suggests that ASICs represent essential sensors for tissue acidosis-related pain. This review provides an update on the role of ASICs in pain sensation and discusses their therapeutic potential for pain management.

Two residues in the extracellular domain convert a nonfunctional ASIC1 into a proton-activated channel

Acid-sensing ion channels (ASICs) are proton-activated sodium channels of the nervous system. Mammals express four ASICs, and orthologs of these genes have been found in all chordates examined to date. Despite a high degree of sequence conservation of all ASICs across species, the response to a given increase in external proton concentration varies markedly: from large and slowly inactivating inward currents to no detectable currents. The underlying bases of this functional variability and whether it stems from differences in proton-binding sites or in structures that translate conformational changes have not been determined yet. We show here that the ASIC1 ortholog of an early vertebrate, lamprey ASIC1, does not respond to protons; however, only two amino acid substitutions for the corresponding ones in rat ASIC1, Q77L and T85L, convert lamprey ASIC1 into a highly sensitive proton-activated channel with apparent H(+) affinity of pH(50) 7.2. Addition of C73H increases the magnitude of the currents by fivefold, and W64R confers desensitization similar to that of the mammalian counterpart. Most amino acid substitutions in these four positions increase the rates of opening and closing the pore, whereas only few, namely, the ones in rat ASIC1, slow the rates. The four residues are located in a contiguous segment made by the beta1-beta2-linker, beta1-strand, and the external segment of the first transmembrane helix. We conclude that the segment thus defined modulates the kinetics of opening and closing the pore and that fast kinetics of desensitization rather than lack of acid sensor accounts for the absence of proton-induced currents in the parent lamprey ASIC1.

Activation of acid-sensing ion channel 1a (ASIC1a) by surface trafficking

Acid-sensing ion channels (ASICs) are voltage-independent Na(+) channels activated by extracellular protons. ASIC1a is expressed in neurons in mammalian brain and is implicated in long term potentiation of synaptic transmission that contributes to learning and memory. In ischemic brain injury, however, activation of this Ca(2+)-permeable channel plays a critical role in acidosis-mediated, glutamate-independent, Ca(2+) toxicity. We report here the identification of insulin as a regulator of ASIC1a surface expression. In modeled ischemia using Chinese hamster ovary cells, serum depletion caused a significant increase in ASIC1a surface expression that resulted in the potentiation of ASIC1a activity. Among the components of serum, insulin was identified as the key factor that maintains a low level of ASIC1a on the plasma membrane. Neurons subjected to insulin depletion increased surface expression of ASIC1a with resultant potentiation of ASIC1a currents. Intracellularly, ASIC1a is predominantly localized to the endoplasmic reticulum in Chinese hamster ovary cells, and this intracellular localization is also observed in neurons. Under conditions of serum or insulin depletion, the intracellular ASIC1a is translocated to the cell surface, increasing the surface expression level. These results reveal an important trafficking mechanism of ASIC1a that is relevant to both the normal physiology and the pathological activity of this channel.

Regulation of acid signaling in rat pulmonary sensory neurons by protease-activated receptor-2

Airway acidification has been consistently observed in airway inflammatory conditions and is known to cause cardiorespiratory symptoms that are, at least in part, mediated through the activation of bronchopulmonary C fibers and the subsequent reflexes. Protease-activated receptor-2 (PAR(2)) is expressed in a variety of cells in the lung and airways and is believed to play a role in airway inflammation and hyperresponsiveness. This study was carried out to investigate the effect of PAR(2) activation on the acid signaling in rat bronchopulmonary C-fiber sensory neurons. Our RT-PCR results revealed the expression of mRNAs for transient receptor potential vanilloid receptor 1 (TRPV1) and four functional acid-sensing ion channel (ASIC) subunits 1a, 1b, 2a, and 3 in these sensory neurons. Preincubation of SLIGRL-NH(2), a specific PAR(2)-activating peptide, markedly enhanced the Ca(2+) transient evoked by extracellular acidification. Pretreatment with PAR(2) agonists significantly potentiated both acid-evoked ASIC- and TRPV1-like whole cell inward currents. Activation of PAR(2) also potentiated the excitability of these neurons to acid, but not electrical stimulation. In addition, the potentiation of acid-evoked responses was not prevented by inhibiting either PLC or PKC nor was mimicked by activation of PKC. In conclusion, activation of PAR(2) modulates the acid signaling in pulmonary sensory neurons, and the interaction may play a role in the pathogenesis of airway inflammatory conditions, where airway acidification and PAR(2) activation can occur simultaneously.

Acid-sensing ionic-channel functional expression in the vestibular endorgans

In the vestibular system, the electrical discharge of the afferent neurons has been found to be highly sensitive to external pH changes, and acid-sensing ionic-channels (ASIC) have been found to be functionally expressed in afferent neurons. No previous attempt to assay the ASIC function in vestibular afferent neurons has been done. In our work we studied the electrical discharge of the afferent neuron of the isolated inner ear of the axolotl (Ambystoma tigrinum) to determine the participation of proton-gated currents in the postransductional information processing in the vestibular system. Microperfusion of FMRF-amide significantly increased the resting activity of the afferent neurons of the semicircular canal indicating that ASIC currents are tonically active in the resting condition. The use of ASIC antagonists, amiloride and acetylsalicylic acid (ASA), significantly reduced the vestibular-nerve discharge, corroborating the idea that the afferent neurons of the vestibular system express ASICs that are sensitive to amiloride, ASA, and to FMRF-amide. The sensitivity of the vestibular afferent-resting discharge to the microperfusion of ASIC acting agents indicates the participation of these currents in the establishment of the afferent-resting discharge.

Dorsal root ganglion neurons innervating skeletal muscle respond to physiological combinations of protons, ATP, and lactate mediated by ASIC, P2X, and TRPV1

The adequate stimuli and molecular receptors for muscle metaboreceptors and nociceptors are still under investigation. We used calcium imaging of cultured primary sensory dorsal root ganglion (DRG) neurons from C57Bl/6 mice to determine candidates for metabolites that could be the adequate stimuli and receptors that could detect these stimuli. Retrograde DiI labeling determined that some of these neurons innervated skeletal muscle. We found that combinations of protons, ATP, and lactate were much more effective than individually applied compounds for activating rapid calcium increases in muscle-innervating dorsal root ganglion neurons. Antagonists for P2X, ASIC, and TRPV1 receptors suggested that these three receptors act together to detect protons, ATP, and lactate when presented together in physiologically relevant concentrations. Two populations of muscle-innervating DRG neurons were found. One responded to low metabolite levels (likely nonnoxious) and used ASIC3, P2X5, and TRPV1 as molecular receptors to detect these metabolites. The other responded to high levels of metabolites (likely noxious) and used ASIC3, P2X4, and TRPV1 as their molecular receptors. We conclude that a combination of ASIC, P2X5 and/or P2X4, and TRPV1 are the molecular receptors used to detect metabolites by muscle-innervating sensory neurons. We further conclude that the adequate stimuli for muscle metaboreceptors and nociceptors are combinations of protons, ATP, and lactate.

Heterologous expression of the invertebrate FMRFamide-gated sodium channel as a mechanism to selectively activate mammalian neurons

Considerable effort has been directed toward the development of methods to selectively activate specific subtypes of neurons. Focus has been placed on the heterologous expression of proteins that are capable of exciting neurons in which they are expressed. Here we describe the heterologous expression of the invertebrate FMRFamide (H-phenylalanine-methionine-arginine-phenylalanine-NH2) -gated sodium channel from Helix aspersa (HaFaNaC) in hippocampal slice cultures. HaFaNaC was co-expressed with a fluorescent protein (green fluorescent protein (GFP), red fluorescent protein from Discosoma sp (dsRed) or mutated form of red fluorescent protein from Discosoma sp (tdTomato)) in CA3 pyramidal neurons of rat hippocampal slice cultures using single cell electroporation. Pressure application of the agonist FMRFamide to HaFaNaC-expressing neuronal somata produced large prolonged depolarizations and bursts of action potentials (APs). FMRFamide responses were inhibited by amiloride (100 microM). In contrast, pressure application of FMRFamide to the axons of neurons expressing HaFaNaC produced no response. Fusion of GFP to the N-terminus of HaFaNaC showed that GFP-HaFaNaC was absent from axons. Bath application of FMRFamide produced persistent AP firing in HaFaNaC-expressing neurons. This FMRFamide-induced increase in the frequency of APs was dose-dependent. The concentrations of FMRFamide required to activate HaFaNaC-expressing neurons were below that required to activate the homologous acid sensing ion channel normally found in mammalian neurons. Furthermore, the mammalian neuropeptides neuropeptide FF and RFamide-related peptide-1, which have amidated RF C-termini, did not affect HaFaNaC-expressing neurons. Antagonists of NPFF receptors (BIBP3226) also had no effect on HaFaNaC. Therefore, we suggest that heterologous-expression of HaFaNaC in mammalian neurons could be a useful method to selectively and persistently excite specific subtypes of neurons in intact nervous tissue.

Modulatory role of neuropeptide FF system in nociception and opiate analgesia

The tetra-peptide FMRF-NH(2) is a cardioexcitatory peptide in the clam. Using the antibody against this peptide, FMRF-NH(2)-like immunoreactive material was detected in mammalian CNS. Subsequently, mammalian FMRF-NH(2) immunoreactive peptides were isolated from bovine brain and characterized to be FLFQPQRF-NH(2) (NPFF) and AGEGLSSPFWSLAAPQRF-NH(2) (NPAF). The genes encoding NPFF precursor proteins and NPFF receptors 1 and 2 are expressed in all vertebrate species examined to date and are highly conserved. Among many biological roles suggested for the NPFF system, the possible modulatory role of NPFF in nocicetion and opiate analgesia has been most widely investigated. Pharmacologically, NPFF-related peptides were found to exhibit analgesia and also potentiate the analgesic activity of opiates when administered intrathecally but attenuate the opiate induced analgesia when administered intracerebroventricularly. RF-NH(2) peptides including NPFF-related peptides were found to delay the rate of acid sensing ion channels (ASIC) desensitization resulting in enhancing acid gated currents, raising the possibility that NPFF also may have a pain modulatory role through ASIC. The genes for NPFF as well as NPFF-R2, preferred receptor for NPFF, are highly unevenly expressed in the rat CNS with the highest levels localized to the superficial layers of the dorsal spinal cord. These two genes are also present in the dorsal root ganglia (DRG), though at low levels in normal rats. NPFF and NPFF-R2 mRNAs were found to be coordinately up-regulated in spinal cord and DRG of rats with peripheral inflammation. In addition, NPFF-R2 immunoreactivity in the primary afferents was increased by peripheral inflammation. The findings from the early studies on the analgesic and morphine modulating activities suggested a role for NPFF in pain modulation and this possibility is further supported by the distribution of NPFF and its receptor and the regulation of the NPFF system in vivo.

Overexpression of neurotrophin-3 enhances the mechanical response properties of slowly adapting type 1 afferents and myelinated nociceptors

Constitutive overexpression of neurotrophin-3 (NT3) in murine skin results in an increased number of sensory neurons within the dorsal root ganglia, an increase of myelinated axons in cutaneous nerves, hyperinnervation of the skin, and an increased number of Merkel cells found in flank skin. Here we used a saphenous skin/nerve preparation to determine if these anatomical changes affect the functional response characteristics of cutaneous sensory neurons. Overexpression of NT3 significantly increased the responses of slowly adapting type 1 (SA1) low-threshold mechanoreceptors and Adelta high-threshold mechanoreceptors to suprathreshold mechanical stimulation. It also resulted in significantly faster conduction velocities of SA1 fibers. In contrast to earlier findings in flank skin, no differences were noted in the numbers of Merkel cells in the touch domes in hindlimb skin of NT3-overexpressing mice. In addition, the number of dermal Merkel cells, located around hair follicles on the dorsum of the foot, was reduced by 55%. The increase in mechanical sensitivity was found to correlate with significant increases in the expression of acid-sensing ion channels (ASIC) 1 and 3. Additional experiments using intracellular recordings and staining procedures confirmed that at least some cutaneous myelinated nociceptors and SA1 mechanoreceptors stained positively for both trkC and ASIC3. These results indicate that cutaneous NT3 overexpression alters the response properties of specific cutaneous sensory neurons, and that these changes may be due to the modulation of putative mechanosensitive ion channels.

Modulation of acid-sensing ion channel activity by nitric oxide

Acid-sensing ion channels (ASICs) are a class of ion channels activated by extracellular protons and are believed to mediate the pain caused by tissue acidosis. Although ASICs have been widely studied, little is known about their regulation by inflammatory mediators. Here, we provide evidence that nitric oxide (NO) potentiates the activity of ASICs. Whole-cell patch-clamp recordings were performed on neonatal rat cultured dorsal root ganglion neurons and on ASIC isoforms expressed in CHO cells. The NO donor S-nitroso-N-acetylpenicillamine (SNAP) potentiates proton-gated currents in DRG neurons and proton-gated currents in CHO cells expressing each of the acid-sensitive ASIC subunits. Modulators of the cGMP/PKG pathway had no effect on the potentiation, but in excised patches from CHO cells expressing ASIC2a, the potentiation could be reversed by externally applied reducing agents. NO therefore has a direct external effect on the ASIC ion channel, probably through oxidization of cysteine residues. Complementary psychophysiological studies were performed using iontophoresis of acidic solutions through the skin of human volunteers. Topical application of the NO donor glyceryl trinitrate significantly increased acid-evoked pain but did not affect heat or mechanical pain thresholds. ASICs may therefore play an important role in the pain associated with metabolic stress and inflammation, where both tissue acidosis and a high level of NO are present.

Heteromeric Assembly of Acid-sensitive Ion Channel and Epithelial Sodium Channel Subunits

Amiloride-sensitive ion channels are formed from homo- or heteromeric combinations of subunits from the epithelial Na+ channel (ENaC)/degenerin superfamily, which also includes the acid-sensitive ion channel (ASIC) family. These channel subunits share sequence homology and topology. In this study, we have demonstrated, using confocal fluorescence resonance energy transfer microscopy and co-immunoprecipitation, that ASIC and ENaC subunits are capable of forming cross-clade intermolecular interactions. We have also shown that combinations of ASIC1 with ENaC subunits exhibit novel electrophysiological characteristics compared with ASIC1 alone. The results of this study suggest that heteromeric complexes of ASIC and ENaC subunits may underlie the diversity of amiloride-sensitive cation conductances observed in a wide variety of tissues and cell types where co-expression of ASIC and ENaC subunits has been observed.

Potentiation of acid-sensing ion channels by sulfhydryl compounds

The acid-sensing ion channels (ASICs) are voltage-independent ion channels activated by acidic extracellular pH. ASICs play a role in sensory transduction, behavior, and acidotoxic neuronal death, which occurs during stroke and ischemia. During these conditions, the extracellular concentration of sulfhydryl reducing agents increases. We used perforated patch-clamp technique to analyze the impact of sulfhydryls on H(+)-gated currents from Chinese hamster ovary (CHO) cells expressing human ASIC1a (hASIC1a). We found that hASIC1a currents activated by pH 6.5 were increased almost twofold by the sulfhydryl-containing reducing agents dithiothreitol (DTT) and glutathione. DTT shifted the pH-dose response of hASIC1a toward a more neutral pH (pH(0.5) from 6.54 to 6.69) and slowed channel desensitization. The effect of reducing agents on native mouse hippocampal neurons and transfected mouse ASIC1a was similar. We found that the effect of DTT on hASIC1a was mimicked by the metal chelator TPEN, and mutant hASIC1a channels with reduced TPEN potentiation showed reduced DTT potentiation. Furthermore, the addition of DTT in the presence of TPEN did not result in further increases in current amplitude. These results suggest that the effect of DTT on hASIC1a is due to relief of tonic inhibition by transition metal ions. We found that all ASICs examined remained potentiated following the removal of DTT. This effect was reversed by the oxidizing agent DTNB in hASIC1a, supporting the hypothesis that DTT also impacts ASICs via a redox-sensitive site. Thus sulfhydryl compounds potentiate H(+)-gated currents via two mechanisms, metal chelation and redox modulation of target amino acids.

Peptides inhibitors of acid-sensing ion channels

Acid-sensing ion channels (ASICs) channels are proton-gated cationic channels mainly expressed in central and peripheric nervous system and related to the epithelial amiloride-sensitive Na(+) channels and to the degenerin family of ion channels. ASICs comprise four proteins forming functional channel subunits (ASIC1a, ASIC1b, ASIC2a, and ASIC3) and two proteins (ASIC2b and ASIC4) without yet known activators. Functional channels are activated by external pH variations ranging from pH(0.5) 6.8 to 4.0 and currents are characterized by either rapid kinetics of inactivation (ASIC1a, ASIC1b, ASIC3) or slow kinetics of inactivation (ASIC2a) and sometimes the presence of a plateau phase (ASIC3). ASIC1a and ASIC3, which are expressed in nociceptive neurons, have been implicated in inflammation and knockout mice studies support the role of ASIC3 in various pain processes. ASIC1a seems more related to synaptic plasticity, memory, learning and fear conditioning in the CNS. ASIC2a contributes to hearing in the cochlea, sour taste sensation, and visual transduction in the retina. The pharmacology of ASICs is limited to rather nonselective drugs such as amiloride, nonsteroid anti-inflammatory drugs, and neuropeptides. Recently, two peptides, PcTx1 and APETx2, isolated from a spider and a sea anemone, have been characterized as selective and high-affinity inhibitors for ASIC1a and ASIC3 channels, respectively. PcTx1 inhibits ASIC1a homomers with an affinity of 0.7 nM (IC(50)) without any effect on ASIC1a containing heteromers and thus helped to characterize ASIC1a homomeric channels in peripheric and central neurons. PcTx1 acts as a gating modifier since it shifts the channel from the resting to an inactivated state by increasing its affinity for H(+). APETx2 is less selective since it inhibits several ASIC3-containing channels (IC(50) from 63 nM to 2 microM) and to date its mode of action is unknown. Nevertheless, APETx2 structure is related to other sea anemone peptides, which act as gating modifiers on Nav and Kv channels.

Distinct ASIC currents are expressed in rat putative nociceptors and are modulated by nerve injury

The H(+)-gated acid-sensing ion channels (ASICs) are expressed in dorsal root ganglion (DRG) neurones. Studies with ASIC knockout mice indicated either a pro-nociceptive or a modulatory role of ASICs in pain sensation. We have investigated in freshly isolated rat DRG neurones whether neurones with different ASIC current properties exist, which may explain distinct cellular roles, and we have investigated ASIC regulation in an experimental model of neuropathic pain. Small-diameter DRG neurones expressed three different ASIC current types which were all preferentially expressed in putative nociceptors. Type 1 currents were mediated by ASIC1a homomultimers and characterized by steep pH dependence of current activation in the pH range 6.8-6.0. Type 3 currents were activated in a similar pH range as type 1, while type 2 currents were activated at pH < 6. When activated by acidification to pH 6.8 or 6.5, the probability of inducing action potentials correlated with the ASIC current density. Nerve injury induced differential regulation of ASIC subunit expression and selective changes in ASIC function in DRG neurones, suggesting a complex reorganization of ASICs during the development of neuropathic pain. In summary, we describe a basis for distinct cellular functions of different ASIC types in small-diameter DRG neurones.

What is the hair cell transduction channel?

In contrast to nearly all other sensory systems, the mechanically sensitive ion channel carrying the receptor current into hair cells of the inner ear has not been identified in molecular terms. A number of candidates from at least two different ion channel families have been considered: these include the epithelial sodium channel (ENaC) and acid-sensing ion channel (ASIC) members of the DEG/ENaC superfamily of amiloride-sensitive sodium channels, as well as the TRP channels TRPN1, TRPV4, TRPML3 and TRPA1. For each, initial supportive results were followed by further studies that cast doubts on their involvement. No promising candidates have recently emerged, but the TRP family continues to be attractive in general.

Peripherally applied neuropeptide SF is equally algogenic in wild type and ASIC3−/− mice

RFa-related peptides play a significant role in the processing of pain in the CNS of mammals. Recently it has been found that, when applied subcutaneously, these peptides elicit a powerful algogenic effect. The question arises whether this peripheral effect can be connected with the ability of RFa-related peptides to decrease the rate of desensitization of acid sensing ionic channels (ASICs) expressed in primary sensory neurons. We have addressed this question by comparing the effects of neuropeptide SF (NPSF), mammalian RFa peptide, in ASIC3-/- and wild-type C57BL/6J mice. Knockout of ASIC3 gene results in the changes in some of the behavioral parameters. However, subcutaneous injections of the NPSF into the n.saphenous innervation area result in a clearly nociceptive behavior in both strains of mice. There is no significant difference in the total time of licking of injected paw in the ASIC3-/- (194+/-22s) and C57BL/6J (227+/-25s) animals. Thus peripheral algogenic effects of NPSF cannot be explained only in terms of their action on the ASIC3 channels and involves some other, still unidentified mechanism.

NMR analysis of Caenorhabditis elegans FLP-18 neuropeptides: implications for NPR-1 activation

Phe-Met-Arg-Phe-NH2 (FMRFamide)-like peptides (FLPs) are the largest neuropeptide family in animals, particularly invertebrates. FLPs are characterized by a C-N-terminal gradient of decreasing amino acid conservation. Neuropeptide receptor 1 (NPR-1) is a G-protein coupled receptor (GPCR), which has been shown to be a strong regulator of foraging behavior and aggregation responses in Caenorhabditis elegans. Recently, ligands for NPR-1 were identified as neuropeptides coded by the precursor genes flp-18 and flp-21 in C. elegans. The flp-18 gene encodes eight FLPs including DFDGAMPGVLRF-NH2 and EMPGVLRF-NH2. These peptides exhibit considerably different activities on NPR-1, with the longer one showing a lower potency. We have used nuclear magnetic resonance and biological activity to investigate structural features that may explain these activity differences. Our data demonstrate that long-range electrostatic interactions exist between N-terminal aspartates and the C-terminal penultimate arginine as well as N-terminal hydrogen-bonding interactions that form transient loops within DFDGAMPGVLRF-NH2. We hypothesize that these loops, along with peptide charge, diminish the activity of this peptide on NPR-1 relative to that of EMPGVLRF-NH2. These results provide some insight into the large amino acid diversity in FLPs.

Acid-sensing ionic channels in the rat vestibular endorgans and ganglia

Acid-sensing ionic channels (ASICs) are members of the epithelial Na+ channel/degenerin (ENaC/DEG) superfamily. ASICs are widely distributed in the central and peripheral nervous system. They have been implicated in synaptic transmission, pain perception, and the mechanoreception in peripheral tissues. Our objective was to characterize proton-gated currents mediated by ASICs and to determine their immunolocation in the rat vestibular periphery. Voltage clamp of cultured afferent neurons from P7 to P10 rats showed a proton-gated current with rapid activation and complete desensitization, which was carried almost exclusively by sodium ions. The current response to protons (H+) has a pH0.5 of 6.2. This current was reversibly decreased by amiloride, gadolinium, lead, acetylsalicylic acid, and enhanced by FMRFamide and zinc, and negatively modulated by raising the extracellular calcium concentration. Functional expression of the current was correlated with smaller-capacitance neurons. Acidification of the extracellular pH generated action potentials in vestibular neurons, suggesting a functional role of ASICs in their excitability. Immunoreactivity to ASIC1a and ASIC2a subunits was found in small vestibular ganglion neurons and afferent fibers that run throughout the macula utricle and crista stroma. ASIC2b, ASIC3, and ASIC4 were expressed to a lesser degree in vestibular ganglion neurons. The ASIC1b subunit was not detected in the vestibular endorgans. No acid-pH-sensitive currents or ASIC immunoreactivity was found in hair cells. Our results indicate that proton-gated current is carried through ASICs and that ionic current activated by H+ contributes to shape the vestibular afferent neurons' response to its synaptic input.

FMRFamide-gated sodium channel and ASIC channels: a new class of ionotropic receptors for FMRFamide and related peptides

FMRFamide and related peptides typically exert their action through G-protein coupled receptors. However, two ionotropic receptors for these peptides have recently been identified. They are both members of the epithelial amiloride-sensitive Na+ channel and degenerin (ENaC/DEG) family of ion channels. The invertebrate FMRFamide-gated Na+ channel (FaNaC) is a neuronal Na+-selective channel which is directly gated by micromolar concentrations of FMRFamide and related tetrapeptides. Its response is fast and partially desensitizing, and FaNaC has been proposed to participate in peptidergic neurotransmission. On the other hand, mammalian acid-sensing ion channels (ASICs) are not gated but are directly modulated by FMRFamide and related mammalian peptides like NPFF and NPSF. ASICs are activated by external protons and are therefore extracellular pH sensors. They are expressed both in the central and peripheral nervous system and appear to be involved in many physiological and pathophysiological processes such as hippocampal long-term potentiation and defects in learning and memory, acquired fear-related behavior, retinal function, brain ischemia, pain sensation in ischemia and inflammation, taste perception, hearing functions, and mechanoperception. The potentiation of ASIC activity by endogenous RFamide neuropeptides probably participates in the response to noxious acidosis in sensory and central neurons. Available data also raises the possibility of the existence of still unknown FMRFamide related endogenous peptides acting as direct agonists for ASICs.