Regulation of ASIC activity by ASIC4--new insights into ASIC channel function revealed by a yeast two-hybrid assay

Acidosis-related pain and its receptors as targets for chronic pain

Sensing acidosis is an important somatosensory function in responses to ischemia, inflammation, and metabolic alteration. Accumulating evidence has shown that acidosis is an effective factor for pain induction and that many intractable chronic pain diseases are associated with acidosis signaling. Various receptors have been known to detect extracellular acidosis and all express in the somatosensory neurons, such as acid sensing ion channels (ASIC), transient receptor potential (TRP) channels and proton-sensing G-protein coupled receptors. In addition to sense noxious acidic stimulation, these proton-sensing receptors also play a vital role in pain processing. For example, ASICs and TRPs are involved in not only nociceptive activation but also anti-nociceptive effects as well as some other non-nociceptive pathways. Herein, we review recent progress in probing the roles of proton-sensing receptors in preclinical pain research and their clinical relevance. We also propose a new concept of sngception to address the specific somatosensory function of acid sensation. This review aims to connect these acid-sensing receptors with basic pain research and clinical pain diseases, thus helping with better understanding the acid-related pain pathogenesis and their potential therapeutic roles via the mechanism of acid-mediated antinociception.

Rubbing Salt in the Wound: Molecular Evolutionary Analysis of Pain-Related Genes Reveals the Pain Adaptation of Cetaceans in Seawater

Pain, usually caused by a strong or disruptive stimulus, is an unpleasant sensation that serves as a warning to organisms. To adapt to extreme environments, some terrestrial animals have evolved to be inherently insensitive to pain. Cetaceans are known as supposedly indifferent to pain from soft tissue injury representatives of marine mammals. However, the molecular mechanisms that explain how cetaceans are adapted to pain in response to seawater environment remain unclear. Here, we performed a molecular evolutionary analysis of pain-related genes in selected representatives of cetaceans. ASIC4 gene was identified to be pseudogenized in all odontocetes (toothed whales) except from Physeter macrocephalus (sperm whales), and relaxed selection of this gene was detected in toothed whales with pseudogenized ASIC4. In addition, positive selection was detected in pain perception (i.e., ASIC3, ANO1, CCK, and SCN9A) and analgesia (i.e., ASIC3, ANO1, CCK, and SCN9A) genes among the examined cetaceans. In this study, potential convergent amino acid substitutions within predicted proteins were found among the examined cetaceans and other terrestrial mammals, inhabiting extreme environments (e.g., V441I of TRPV1 in cetaceans and naked mole rats). Moreover, specific amino acid substitutions within predicted sequences of several proteins were found in the studied representatives of cetaceans (e.g., F56L and D163A of ASIC3, E88G of GRK2, and F159L of OPRD1). Most of the substitutions were located within important functional domains of proteins, affecting their protein functions. The above evidence suggests that cetaceans might have undergone adaptive molecular evolution in pain-related genes through different evolutionary patterns to adapt to pain, resulting in greater sensitivity to pain and more effective analgesia. This study could have implications for diagnosis and treatment of human pain.

Mambalgin-2 Inhibits Lung Adenocarcinoma Growth and Migration by Selective Interaction With ASIC1/α-ENaC/γ-ENaC Heterotrimer

Lung cancer is one of the most common cancer types in the world. Despite existing treatment strategies, overall patient survival remains low and new targeted therapies are required. Acidification of the tumor microenvironment drives the growth and metastasis of many cancers. Acid sensors such as acid-sensing ion channels (ASICs) may become promising targets for lung cancer therapy. Previously, we showed that inhibition of the ASIC1 channels by a recombinant analogue of mambalgin-2 from Dendroaspis polylepis controls oncogenic processes in leukemia, glioma, and melanoma cells. Here, we studied the effects and molecular targets of mambalgin-2 in lung adenocarcinoma A549 and Lewis cells, lung transformed WI-38 fibroblasts, and lung normal HLF fibroblasts. We found that mambalgin-2 inhibits the growth and migration of A549, metastatic Lewis P29 cells, and WI-38 cells, but not of normal fibroblasts. A549, Lewis, and WI-38 cells expressed different ASIC and ENaC subunits, while normal fibroblasts did not at all. Mambalgin-2 induced G2/M cell cycle arrest and apoptosis in lung adenocarcinoma cells. In line, acidification-evoked inward currents were observed only in A549 and WI-38 cells. Gene knockdown showed that the anti-proliferative and anti-migratory activity of mambalgin-2 is dependent on the expression of ASIC1a, α-ENaC, and γ-ENaC. Using affinity extraction and immunoprecipitation, mambalgin-2 targeting of ASIC1a/α-ENaC/γ-ENaC heteromeric channels in A549 cells was shown. Electrophysiology studies in Xenopus oocytes revealed that mambalgin-2 inhibits the ASIC1a/α-ENaC/γ-ENaC channels with higher efficacy than the ASIC1a channels, pointing on the heteromeric channels as a primary target of the toxin in cancer cells. Finally, bioinformatics analysis showed that the increased expression of ASIC1 and γ-ENaC correlates with a worse survival prognosis for patients with lung adenocarcinoma. Thus, the ASIC1a/α-ENaC/γ-ENaC heterotrimer can be considered a marker of cell oncogenicity and its targeting is promising for the design of new selective cancer therapeutics.

Alterations in the Ca2+ toolkit in oesophageal adenocarcinoma

Aim:

To investigate alterations in transcription of genes, encoding Ca²⁺ toolkit proteins, in oesophageal adenocarcinoma (OAC) and to assess associations between gene expression, tumor grade, nodal-metastatic stage, and patient survival.

Methods:

The expression of 275 transcripts, encoding components of the Ca²⁺ toolkit, was analyzed in two OAC datasets: the Cancer Genome Atlas [via the University of Alabama Cancer (UALCAN) portal] and the oesophageal-cancer, clinical, and molecular stratification [Oesophageal Cancer Clinical and Molecular Stratification (OCCAMS)] dataset. Effects of differential expression of these genes on patient survival were determined using Kaplan-Meier log-rank tests. OAC grade- and metastatic-stage status was investigated for a subset of genes. Adjustment for the multiplicity of testing was made throughout.

Results:

Of the 275 Ca²⁺-toolkit genes analyzed, 75 displayed consistent changes in expression between OAC and normal tissue in both datasets. The channel-encoding genes, N-methyl-D-aspartate receptor 2D (GRIN2D), transient receptor potential (TRP) ion channel classical or canonical 4 (TRPC4), and TRP ion channel melastatin 2 (TRPM2) demonstrated the greatest increase in expression in OAC in both datasets. Nine genes were consistently upregulated in both datasets and were also associated with improved survival outcomes. The 6 top-ranking genes for the weighted significance of altered expression and survival outcomes were selected for further analysis: voltage-gated Ca²⁺ channel subunit α 1D (CACNA1D), voltage-gated Ca²⁺ channel auxiliary subunit α2 δ4 (CACNA2D4), junctophilin 1 (JPH1), acid-sensing ion channel 4 (ACCN4), TRPM5, and secretory pathway Ca²⁺ ATPase 2 (ATP2C2). CACNA1D, JPH1, and ATP2C2 were also upregulated in advanced OAC tumor grades and nodal-metastatic stages in both datasets.

Conclusions:

This study has unveiled alterations of the Ca²⁺ toolkit in OAC, compared to normal tissue. Such Ca²⁺ signalling findings are consistent with those from studies on other cancers. Genes that were consistently upregulated in both datasets might represent useful markers for patient diagnosis. Genes that were consistently upregulated, and which were associated with improved survival, might be useful markers for patient outcome. These survival-associated genes may also represent targets for the development of novel chemotherapeutic agents.

Acid-sensing ion channel 3: An analgesic target

Acid-sensing ion channel 3 (ASIC3) belongs to the epithelial sodium channel/degenerin (ENaC/DEG) superfamily. There are 7 different ASIC subunits encoded by 5 different genes. Most ASIC subunits form trimeric ion channels that upon activation by extracellular protons mediate a transient inward current inducing cellular excitability. ASIC subunits exhibit differential tissue expression and biophysical properties, and the ability of subunits to form homo- and heteromeric trimers further increases the complexity of currents measured and their pharmacological properties. ASIC3 is of particular interest, not only because it exhibits high expression in sensory neurones, but also because upon activation it does not fully inactivate: a transient current is followed by a sustained current that persists during a period of extracellular acidity, i.e. ASIC3 can encode prolonged acidosis as a nociceptive signal. Furthermore, certain mediators sensitize ASIC3 enabling smaller proton concentrations to activate it and other mediators can directly activate the channel at neutral pH. Moreover, there is a plethora of evidence using transgenic mouse models and pharmacology, which supports ASIC3 as being a potential target for development of analgesics. This review will focus on current understanding of ASIC3 function to provide an overview of how ASIC3 contributes to physiology and pathophysiology, examining the mechanisms by which it can be modulated, and highlighting gaps in current understanding and future research directions.

Adult Neural Plasticity in Naked Mole-Rats: Implications of Fossoriality, Longevity and Sociality on the Brain's Capacity for Change

Naked mole-rats (Heterocephalus glaber) are small African rodents that have many unique behavioral and physiological adaptations well-suited for testing hypotheses about mammalian neural plasticity. In this chapter, we focus on three features of naked mole-rat biology and how they impact neural plasticity in this species: (1) their fossorial lifestyle, (2) their extreme longevity with a lack of demonstrable senescence, and (3) their unusual social structure. Critically, each of these features requires some degree of biological flexibility. First, their fossorial habitat situates them in an environment with characteristics to which the central nervous system is particularly sensitive (e.g., oxygen content, photoperiod, spatial complexity). Second, their long lifespan requires adaptations to combat senescence and declines in neural functioning. Finally, their extreme reproductive skew and sustained ability for release from reproductive suppression indicates remarkable neural sensitivity to the sociosexual environment that is distinct from chronological age. These three features of naked mole-rat life are not mutually exclusive, but they do each offer unique considerations for the possibilities, constraints, and mechanisms associated with adult neural plasticity.

Cell-Type-Specific Expression Pattern of Proton-Sensing Receptors and Channels in Pituitary Gland

In mammalian cells, extracellular protons act as orthosteric and allosteric ligands for multiple receptors and channels. The aim of this study is to identify proton sensors in the rat pituitary gland. qRT-PCR analysis indicated the expression of G-protein-coupled receptor 68 gene (Gpr68) and acid-sensing ion channel (ASIC) genes Asic1, Asic2, and Asic4 in anterior pituitary cells and Asic1 and Asic2 in immortalized GH3 pituitary cells. Asic1a and Asic2b were the dominant splice isoforms. Single anterior pituitary cell RNA sequencing and immunocytochemical analysis showed that nonexcitable folliculostellate cells express GPR68 gene and protein, whereas excitable secretory cells express ASIC genes and proteins. Asic1 was detected in all secretory cell types, Asic2 in gonadotrophs, thyrotrophs, and somatotrophs, and Asic4 in lactotrophs. Extracellular acidification activated two types of currents in a concentration-dependent manner: a fast-developing, desensitizing current with an estimated EC₅₀-value of pH 6.7 and a slow-developing, non-desensitizing current that required a higher proton concentration for activation. The desensitizing current was abolished by removal of bath sodium and application of amiloride, a blocker of ASIC channels, whereas the non-desensitizing current was amiloride insensitive and voltage dependent. Activation of both currents increased the excitability of secretory pituitary cells, consistent with their potential physiological relevance in control of voltage-gated calcium influx and calcium-dependent cellular functions.

Animal, Herb, and Microbial Toxins for Structural and Pharmacological Study of Acid-Sensing Ion Channels

Acid-sensing ion channels (ASICs) are of the most sensitive molecular sensors of extracellular pH change in mammals. Six isoforms of these channels are widely represented in membranes of neuronal and non-neuronal cells, where these molecules are involved in different important regulatory functions, such as synaptic plasticity, learning, memory, and nociception, as well as in various pathological states. Structural and functional studies of both wild-type and mutant ASICs are essential for human care and medicine for the efficient treatment of socially significant diseases and ensure a comfortable standard of life. Ligands of ASICs serve as indispensable tools for these studies. Such bioactive compounds can be synthesized artificially. However, to date, the search for such molecules has been most effective amongst natural sources, such as animal venoms or plants and microbial extracts. In this review, we provide a detailed and comprehensive structural and functional description of natural compounds acting on ASICs, as well as the latest information on structural aspects of their interaction with the channels. Many of the examples provided in the review demonstrate the undoubted fundamental and practical successes of using natural toxins. Without toxins, it would not be possible to obtain data on the mechanisms of ASICs' functioning, provide detailed study of their pharmacological properties, or assess the contribution of the channels to development of different pathologies. The selectivity to different isoforms and variety in the channel modulation mode allow for the appraisal of prospective candidates for the development of new drugs.

Machine-Learned Association of Next-Generation Sequencing-Derived Variants in Thermosensitive Ion Channels Genes with Human Thermal Pain Sensitivity Phenotypes

Genetic association studies have shown their usefulness in assessing the role of ion channels in human thermal pain perception. We used machine learning to construct a complex phenotype from pain thresholds to thermal stimuli and associate it with the genetic information derived from the next-generation sequencing (NGS) of 15 ion channel genes which are involved in thermal perception, including ASIC1, ASIC2, ASIC3, ASIC4, TRPA1, TRPC1, TRPM2, TRPM3, TRPM4, TRPM5, TRPM8, TRPV1, TRPV2, TRPV3, and TRPV4. Phenotypic information was complete in 82 subjects and NGS genotypes were available in 67 subjects. A network of artificial neurons, implemented as emergent self-organizing maps, discovered two clusters characterized by high or low pain thresholds for heat and cold pain. A total of 1071 variants were discovered in the 15 ion channel genes. After feature selection, 80 genetic variants were retained for an association analysis based on machine learning. The measured performance of machine learning-mediated phenotype assignment based on this genetic information resulted in an area under the receiver operating characteristic curve of 77.2%, justifying a phenotype classification based on the genetic information. A further item categorization finally resulted in 38 genetic variants that contributed most to the phenotype assignment. Most of them (10) belonged to the TRPV3 gene, followed by TRPM3 (6). Therefore, the analysis successfully identified the particular importance of TRPV3 and TRPM3 for an average pain phenotype defined by the sensitivity to moderate thermal stimuli.

Evolution of acid nociception: ion channels and receptors for detecting acid

Nociceptors, i.e. sensory neurons tuned to detect noxious stimuli, are found in numerous phyla of the Animalia kingdom and are often polymodal, responding to a variety of stimuli, e.g. heat, cold, pressure and chemicals, such as acid. Owing to the ability of protons to have a profound effect on ionic homeostasis and damage macromolecular structures, it is no wonder that the ability to detect acid is conserved across many species. To detect changes in pH, nociceptors are equipped with an assortment of different acid sensors, some of which can detect mild changes in pH, such as the acid-sensing ion channels, proton-sensing G protein-coupled receptors and several two-pore potassium channels, whereas others, such as the transient receptor potential vanilloid 1 ion channel, require larger shifts in pH. This review will discuss the evolution of acid sensation and the different mechanisms by which nociceptors can detect acid. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.

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.

The synaptic action of Degenerin/Epithelial sodium channels

Degenerin/Epithelial Sodium Channels (DEG/ENaCs) are a large family of animal-specific non-voltage gated ion channels, with enriched expression in neuronal and epithelial tissues. While neuronal DEG/ENaCs were originally characterized as sensory receptor channels, recent studies indicate that several DEG/ENaC family members are also expressed throughout the central nervous system. Human genome-wide association studies have linked DEG/ENaC-coding genes with several neurologic and psychiatric disorders, including epilepsy and panic disorder. In addition, studies in rodent models further indicate that DEG/ENaC activity in the brain contributes to many behaviors, including those related to anxiety and long-term memory. Although the exact neurophysiological functions of DEG/ENaCs remain mostly unknown, several key studies now suggest that multiple family members might exert their neuronal function via the direct modulation of synaptic processes. Here, we review and discuss recent findings on the synaptic functions of DEG/ENaCs in both vertebrate and invertebrate species, and propose models for their possible roles in synaptic physiology.

Walnut oil improves spatial memory in rats and increases the expression of acid-sensing ion channel genes Asic2a and Asic4

Although Walnut oil (WO) has been reported to enhance cognitive function, the underlying molecular mechanisms are not well understood. This study was designed to assess the effects of WO on spatial memory in rats through modulation of the expression of acid-sensing ion channel genes, Asic2a and Asic4. To investigate the effect of WO on cognitive performance, we supplemented the diet of female rats with WO. The results showed that supplementation with WO at doses of 2.2 and 11 g kg-1 day-1 significantly improved learning and memory. In vitro treatment of rat hippocampal neuronal cells with appropriate doses of WO revealed a significant increase in the expression of Asic2a and Asic4 in a dose-dependent manner at both the mRNA and protein levels. We conclude that WO intake might help to prevent cognitive decline, particularly in the elderly, and that ASIC genes in neurons can be the targets of compounds contained in the oil.

Protons as Messengers of Intercellular Communication in the Nervous System

In this review, evidence demonstrating that protons (H⁺) constitute a complex, regulated intercellular signaling mechanisms are presented. Given that pH is a strictly regulated variable in multicellular organisms, localized extracellular pH changes may constitute significant signals of cellular processes that occur in a cell or a group of cells. Several studies have demonstrated that the low pH of synaptic vesicles implies that neurotransmitter release is always accompanied by the co-release of H⁺ into the synaptic cleft, leading to transient extracellular pH shifts. Also, evidence has accumulated indicating that extracellular H⁺ concentration regulation is complex and implies a source of protons in a network of transporters, ion exchangers, and buffer capacity of the media that may finally establish the extracellular proton concentration. The activation of membrane transporters, increased production of CO₂ and of metabolites, such as lactate, produce significant extracellular pH shifts in nano- and micro-domains in the central nervous system (CNS), constituting a reliable signal for intercellular communication. The acid sensing ion channels (ASIC) function as specific signal sensors of proton signaling mechanism, detecting subtle variations of extracellular H⁺ in a range varying from pH 5 to 8. The main question in relation to this signaling system is whether it is only synaptically restricted, or a volume modulator of neuron excitability. This signaling system may have evolved from a metabolic activity detection mechanism to a highly localized extracellular proton dependent communication mechanism. In this study, evidence showing the mechanisms of regulation of extracellular pH shifts and of the ASICs and its function in modulating the excitability in various systems is reviewed, including data and its role in synaptic neurotransmission, volume transmission and even segregated neurotransmission, leading to a reliable extracellular signaling mechanism.

Acid-sensing ion channels emerged over 600 Mya and are conserved throughout the deuterostomes

Acid-sensing ion channels (ASICs) are proton-gated ion channels broadly expressed in the vertebrate nervous system, converting decreased extracellular pH into excitatory sodium current. ASICs were previously thought to be a vertebrate-specific branch of the DEG/ENaC family, a broadly conserved but functionally diverse family of channels. Here, we provide phylogenetic and experimental evidence that ASICs are conserved throughout deuterostome animals, showing that ASICs evolved over 600 million years ago. We also provide evidence of ASIC expression in the central nervous system of the tunicate, Oikopleura dioica Furthermore, by comparing broadly related ASICs, we identify key molecular determinants of proton sensitivity and establish that proton sensitivity of the ASIC4 isoform was lost in the mammalian lineage. Taken together, these results suggest that contributions of ASICs to neuronal function may also be conserved broadly in numerous animal phyla.

Acid-sensing ion channels: dual function proteins for chemo-sensing and mechano-sensing

Background

Acid-sensing ion channels (ASICs) are a group of amiloride-sensitive ligand-gated ion channels belonging to the family of degenerin/epithelial sodium channels. ASICs are predominantly expressed in both the peripheral and central nervous system and have been characterized as potent proton sensors to detect extracellular acidification in the periphery and brain.

Main body

Here we review the recent studies focusing on the physiological roles of ASICs in the nervous system. As the major acid-sensing membrane proteins in the nervous system, ASICs detect tissue acidosis occurring at tissue injury, inflammation, ischemia, stroke, and tumors as well as fatiguing muscle to activate pain-sensing nerves in the periphery and transmit pain signals to the brain. Arachidonic acid and lysophosphocholine have been identified as endogenous non-proton ligands activating ASICs in a neutral pH environment. On the other hand, ASICs are found involved in the tether model mechanotransduction, in which the extracellular matrix and cytoplasmic cytoskeletons act like a gating-spring to tether the mechanically activated ion channels and thus transmit the stimulus force to the channels. Accordingly, accumulating evidence has shown ASICs play important roles in mechanotransduction of proprioceptors, mechanoreceptors and nociceptors to monitor the homoeostatic status of muscle contraction, blood volume, and blood pressure as well as pain stimuli.

Conclusion

Together, ASICs are dual-function proteins for both chemosensation and mechanosensation involved in monitoring physiological homoeostasis and pathological signals.

Naked mole-rat acid-sensing ion channel 3 forms nonfunctional homomers, but functional heteromers

Acid-sensing ion channels (ASICs) form both homotrimeric and heterotrimeric ion channels that are activated by extracellular protons and are involved in a wide range of physiological and pathophysiological processes, including pain and anxiety. ASIC proteins can form both homotrimeric and heterotrimeric ion channels. The ASIC3 subunit has been shown to be of particular importance in the peripheral nervous system with pharmacological and genetic manipulations demonstrating a role in pain. Naked mole-rats, despite having functional ASICs, are insensitive to acid as a noxious stimulus and show diminished avoidance of acidic fumes, ammonia, and carbon dioxide. Here we cloned naked mole-rat ASIC3 (nmrASIC3) and used a cell-surface biotinylation assay to demonstrate that it traffics to the plasma membrane, but using whole-cell patch clamp electrophysiology we observed that nmrASIC3 is insensitive to both protons and the non-proton ASIC3 agonist 2-guanidine-4-methylquinazoline. However, in line with previous reports of ASIC3 mRNA expression in dorsal root ganglia neurons, we found that the ASIC3 antagonist APETx2 reversibly inhibits ASIC-like currents in naked mole-rat dorsal root ganglia neurons. We further show that like the proton-insensitive ASIC2b and ASIC4, nmrASIC3 forms functional, proton-sensitive heteromers with other ASIC subunits. An amino acid alignment of ASIC3s between 9 relevant rodent species and human identified unique sequence differences that might underlie the proton insensitivity of nmrASIC3. However, introducing nmrASIC3 differences into rat ASIC3 (rASIC3) produced only minor differences in channel function, and replacing the nmrASIC3 sequence with that of rASIC3 did not produce a proton-sensitive ion channel. Our observation that nmrASIC3 forms nonfunctional homomers may reflect a further adaptation of the naked mole-rat to living in an environment with high-carbon dioxide levels.

Acid-Sensing Ion Channels as Potential Therapeutic Targets in Neurodegeneration and Neuroinflammation

Acid-sensing ion channels (ASICs) are a family of proton-sensing channels that are voltage insensitive, cation selective (mostly permeable to Na⁺), and nonspecifically blocked by amiloride. Derived from 5 genes (ACCN1-5), 7 subunits have been identified, 1a, 1b, 2a, 2b, 3, 4, and 5, that are widely expressed in the peripheral and central nervous system as well as other tissues. Over the years, different studies have shown that activation of these channels is linked to various physiological and pathological processes, such as memory, learning, fear, anxiety, ischemia, and multiple sclerosis to name a few, so their potential as therapeutic targets is increasing. This review focuses on recent advances that have helped us to better understand the role played by ASICs in different pathologies related to neurodegenerative diseases, inflammatory processes, and pain.

Distribution of ASIC4 transcripts in the adult wild-type mouse brain

Acid-sensing ion channel 4 (ASIC4) belongs to the ASIC gene family of neuronal proton-gated cation channels, and is the least understood subtype among the members. Previous studies of ASIC4 expression in the mammalian central nervous system have shown that ASIC4 is abundantly expressed in the spinal cord and in various brain regions, such as the cerebral cortex, the hippocampus, and the cerebellum. However, the detailed distribution of ASIC4 transcripts in mammalian brains still remains to be elucidated. In the present study, radioactive in situ hybridization histochemistry with an ASIC4-specific cRNA probe was performed on wild-type mouse brains, followed by X-gal staining experiments with Asic4-lacZ reporter mice Asic4^{tm1a(KOMP)Mbp}. It was found that ASIC4 mRNAs were widely expressed throughout the wild-type brain, but preferentially concentrated in the olfactory bulb, the piriform cortex, the caudate putamen, the preoptic area, the paraventricular nucleus, the medial habenular nucleus, the pretectal area, the lateral geniculate nucleus, the amygdaloid complex, the superior colliculus, the interpeduncular nucleus, and the granule cell layer of the ventral hippocampus, and these results were in agreement with the X-gal-positive reactions observed in the mutant brain. In addition, X-gal staining combined with immunohistochemistry identified intense signals for ASIC4 transcriptional activity in most of the choline acetyltransferase (ChAT)-positive principal neurons located in the basal forebrain cholinergic nuclei. Our data provide useful information to speculate possible roles of ASIC4 in diverse brain functions.

Expression of acid-sensing ion channels and selection of reference genes in mouse and naked mole rat

Acid-sensing ion channels (ASICs) are a family of ion channels comprised of six subunits encoded by four genes and they are expressed throughout the peripheral and central nervous systems. ASICs have been implicated in a wide range of physiological and pathophysiological processes: pain, breathing, synaptic plasticity and excitotoxicity. Unlike mice and humans, naked mole-rats do not perceive acid as a noxious stimulus, even though their sensory neurons express functional ASICs, likely an adaptation to living in a hypercapnic subterranean environment. Previous studies of ASIC expression in the mammalian nervous system have often not examined all subunits, or have failed to adequately quantify expression between tissues; to date there has been no attempt to determine ASIC expression in the central nervous system of the naked mole-rat. Here we perform a geNorm study to identify reliable housekeeping genes in both mouse and naked mole-rat and then use quantitative real-time PCR to estimate the relative amounts of ASIC transcripts in different tissues of both species. We identify RPL13A (ribosomal protein L13A) and CANX (calnexin), and β-ACTIN and EIF4A (eukaryotic initiation factor 4a) as being the most stably expressed housekeeping genes in mouse and naked mole-rat, respectively. In both species, ASIC3 was most highly expressed in dorsal root ganglia (DRG), and ASIC1a, ASIC2b and ASIC3 were more highly expressed across all brain regions compared to the other subunits. We also show that ASIC4, a proton-insensitive subunit of relatively unknown function, was highly expressed in all mouse tissues apart from DRG and hippocampus, but was by contrast the lowliest expressed ASIC in all naked mole-rat tissues.

The function and regulation of acid-sensing ion channels (ASICs) and the epithelial Na(+) channel (ENaC): IUPHAR Review 19

Acid-sensing ion channels (ASICs) and the epithelial Na(+) channel (ENaC) are both members of the ENaC/degenerin family of amiloride-sensitive Na(+) channels. ASICs act as proton sensors in the nervous system where they contribute, besides other roles, to fear behaviour, learning and pain sensation. ENaC mediates Na(+) reabsorption across epithelia of the distal kidney and colon and of the airways. ENaC is a clinically used drug target in the context of hypertension and cystic fibrosis, while ASIC is an interesting potential target. Following a brief introduction, here we will review selected aspects of ASIC and ENaC function. We discuss the origin and nature of pH changes in the brain and the involvement of ASICs in synaptic signalling. We expose how in the peripheral nervous system, ASICs cover together with other ion channels a wide pH range as proton sensors. We introduce the mechanisms of aldosterone-dependent ENaC regulation and the evidence for an aldosterone-independent control of ENaC activity, such as regulation by dietary K(+) . We then provide an overview of the regulation of ENaC by proteases, a topic of increasing interest over the past few years. In spite of the profound differences in the physiological and pathological roles of ASICs and ENaC, these channels share many basic functional and structural properties. It is likely that further research will identify physiological contexts in which ASICs and ENaC have similar or overlapping roles.

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.

Acid-sensing ion channels: potential therapeutic targets for neurologic diseases

Maintaining the physiological pH of interstitial fluid is crucial for normal cellular functions. In disease states, tissue acidosis is a common pathologic change causing abnormal activation of acid-sensing ion channels (ASICs), which according to cumulative evidence, significantly contributes to inflammation, mitochondrial dysfunction, and other pathologic mechanisms (i.e., pain, stroke, and psychiatric conditions). Thus, it has become increasingly clear that ASICs are critical in the progression of neurologic diseases. This review is focused on the importance of ASICs as potential therapeutic targets in combating neurologic diseases.

Genetic mapping of ASIC4 and contrasting phenotype to ASIC1a in modulating innate fear and anxiety

Although ASIC4 is a member of the acid-sensing ion channel (ASIC) family, we have limited knowledge of its expression and physiological function in vivo. To trace the expression of this ion channel, we generated the ASIC4-knockout/CreERT(2)-knockin (Asic4(Cre) (ERT) (2)) mouse line. After tamoxifen induction in the Asic4(Cre) (ERT)(2)::CAG-STOP(floxed)-Td-tomato double transgenic mice, we mapped the expression of ASIC4 at the cellular level in the central nervous system (CNS). ASIC4 was expressed in many brain regions, including the olfactory bulb, cerebral cortex, striatum, hippocampus, amygdala, thalamus, hypothalamus, brain stem, cerebellum, spinal cord and pituitary gland. Colocalisation studies further revealed that ASIC4 was expressed mainly in three types of cells in the CNS: (i) calretinin (CR)-positive and/or vasoactive intestine peptide (VIP)-positive interneurons; (ii) neural/glial antigen 2 (NG2)-positive glia, also known as oligodendrocyte precursor cells; and (iii) cerebellar granule cells. To probe the possible role of ASIC4, we hypothesised that ASIC4 could modulate the membrane expression of ASIC1a and thus ASIC1a signaling in vivo. We conducted behavioral phenotyping of Asic4(Cre) (ERT)(2) mice by screening many of the known behavioral phenotypes found in Asic1a knockouts and found ASIC4 not involved in shock-evoked fear learning and memory, seizure termination or psychostimulant-induced locomotion/rewarding effects. In contrast, ASIC4 might play an important role in modulating the innate fear response to predator odor and anxious state because ASIC4-mutant mice showed increased freezing response to 2,4,5-trimethylthiazoline and elevated anxiety-like behavior in both the open-field and elevated-plus maze. ASIC4 may modulate fear and anxiety by counteracting ASIC1a activity in the brain.

Differential regulation of proton-sensitive ion channels by phospholipids: a comparative study between ASICs and TRPV1

Protons are released in pain-generating pathological conditions such as inflammation, ischemic stroke, infection, and cancer. During normal synaptic activities, protons are thought to play a role in neurotransmission processes. Acid-sensing ion channels (ASICs) are typical proton sensors in the central nervous system (CNS) and the peripheral nervous system (PNS). In addition to ASICs, capsaicin- and heat-activated transient receptor potential vanilloid 1 (TRPV1) channels can also mediate proton-mediated pain signaling. In spite of their importance in perception of pH fluctuations, the regulatory mechanisms of these proton-sensitive ion channels still need to be further investigated. Here, we compared regulation of ASICs and TRPV1 by membrane phosphoinositides, which are general cofactors of many receptors and ion channels. We observed that ASICs do not require membrane phosphatidylinositol 4-phosphate (PI(4)P) or phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) for their function. However, TRPV1 currents were inhibited by simultaneous breakdown of PI(4)P and PI(4,5)P₂. By using a novel chimeric protein, CF-PTEN, that can specifically dephosphorylate at the D3 position of phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P₃), we also observed that neither ASICs nor TRPV1 activities were altered by depletion of PI(3,4,5)P₃ in intact cells. Finally, we compared the effects of arachidonic acid (AA) on two proton-sensitive ion channels. We observed that AA potentiates the currents of both ASICs and TRPV1, but that they have different recovery aspects. In conclusion, ASICs and TRPV1 have different sensitivities toward membrane phospholipids, such as PI(4)P, PI(4,5)P₂, and AA, although they have common roles as proton sensors. Further investigation about the complementary roles and respective contributions of ASICs and TRPV1 in proton-mediated signaling is necessary.

Genetic exploration of the role of acid-sensing ion channels

Advanced gene targeting technology and related tools in mice have been incorporated into studies of acid-sensing ion channels (ASICs). A single ASIC subtype can be knocked out specifically and screened thoroughly for expression in the nervous system at the cellular level. Mapping studies have further shed light on the initiation and identification of related behavioral phenotypes. Here we review studies involving genetically engineered mouse models used to investigate the physiological function of individual ASIC subtypes: ASIC1 (and ASIC1a), ASIC2, ASIC3 and ASIC4. We discuss the detailed expression studies and significant phenotypes revealed with gene knockout for most known Asic subtypes. Each strategy designed to manipulate mouse genetics has advantages and disadvantages. We discuss the limitations of these Asic-knockout models and propose future directions to solve the genetic issues. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

Monoamine NMDA receptor channel blockers inhibit and potentiate native and recombinant proton-gated ion channels

Acid-sensing ion channels (ASICs) are widely distributed in the peripheral and central nervous system. Although they are involved in many physiological functions, the actual processes that activate ASICs remain unclear. This is particularly true for brain ASICs, which produce only a transient response to a fast drop in pH and cannot mediate sustained current. Therefore, the search for ASIC inhibitors and, especially, potentiators/activators is important. We report that NMDA receptor channel blockers with a comparatively simple structure (9-aminoacridine, memantine, IEM-2117 and IEM-1921) potentiate and/or inhibit ASICs in submillimolar concentrations. The experiments were performed using the patch clamp technique on native ASICs from rat hippocampal interneurons and recombinant ASICs of different subunit compositions expressed in CHO cells. Native ASICs were potentiated by IEM-1921 and IEM-2117, and inhibited by memantine and 9-aminoacridine. Homomeric ASIC1a were inhibited by memantine, IEM-2117 and 9-aminoacridine while IEM-1921 was ineffective. In contrast, homomeric ASIC2a were potentiated by IEM-2117, memantine and IEM-1921, whereas 9-aminoacridine was inactive. The compounds caused a complex effect on ASIC3. 9-aminoacridine and IEM-1921 potentiated the steady-state response of ASIC3 and inhibited the peak component. IEM-2117 not only potentiated ASIC3-mediated currents caused by acidification but also evoked steady-state currents at neutral pH. Our results demonstrate that, depending on the subunit composition, ASICs can be activated or inhibited by simple compounds that possess only amino group and aromatic/hydrophobic moieties. This opens up the possibility to search for new ASIC modulators among a number of endogenous ligands.

ASIC2 subunits facilitate expression at the cell surface and confer regulation by PSD-95

Acid-sensing ion channels (ASICs) are Na⁺ channels activated by changes in pH within the peripheral and central nervous systems. Several different isoforms of ASICs combine to form trimeric channels, and their properties are determined by their subunit composition. ASIC2 subunits are widely expressed throughout the brain, where they heteromultimerize with their partnering subunit, ASIC1a. However, ASIC2 contributes little to the pH sensitivity of the channels, and so its function is not well understood. We found that ASIC2 increased cell surface levels of the channel when it is coexpressed with ASIC1a, and genetic deletion of ASIC2 reduced acid-evoked current amplitude in mouse hippocampal neurons. Additionally, ASIC2a interacted with the neuronal synaptic scaffolding protein PSD-95, and PSD-95 reduced cell surface expression and current amplitude in ASICs that contain ASIC2a. Overexpression of PSD-95 also reduced acid-evoked current amplitude in hippocampal neurons. This result was dependent upon ASIC2 since the effect of PSD-95 was abolished in ASIC2−/− neurons. These results lend support to an emerging role of ASIC2 in the targeting of ASICs to surface membranes, and allows for interaction with PSD-95 to regulate these processes.

Calcium-permeable ion channels in pain signaling

The detection and processing of painful stimuli in afferent sensory neurons is critically dependent on a wide range of different types of voltage- and ligand-gated ion channels, including sodium, calcium, and TRP channels, to name a few. The functions of these channels include the detection of mechanical and chemical insults, the generation of action potentials and regulation of neuronal firing patterns, the initiation of neurotransmitter release at dorsal horn synapses, and the ensuing activation of spinal cord neurons that project to pain centers in the brain. Long-term changes in ion channel expression and function are thought to contribute to chronic pain states. Many of the channels involved in the afferent pain pathway are permeable to calcium ions, suggesting a role in cell signaling beyond the mere generation of electrical activity. In this article, we provide a broad overview of different calcium-permeable ion channels in the afferent pain pathway and their role in pain pathophysiology.

Acid-sensing ion channels 1a (ASIC1a) inhibit neuromuscular transmission in female mice

Acid-sensing ion channels (ASIC) open in response to extracellular acidosis. ASIC1a, a particular subtype of these channels, has been described to have a postsynaptic distribution in the brain, being involved not only in ischemia and epilepsy, but also in fear and psychiatric pathologies. High-frequency stimulation of skeletal motor nerve terminals (MNTs) can induce presynaptic pH changes in combination with an acidification of the synaptic cleft, known to contribute to muscle fatigue. Here, we studied the role of ASIC1a channels on neuromuscular transmission. We combined a behavioral wire hanging test with electrophysiology, pharmacological, and immunofluorescence techniques to compare wild-type and ASIC1a lacking mice (ASIC1a ^−/− knockout). Our results showed that 1) ASIC1a ^−/− female mice were weaker than wild type, presenting shorter times during the wire hanging test; 2) spontaneous neurotransmitter release was reduced by ASIC1a activation, suggesting a presynaptic location of these channels at individual MNTs; 3) ASIC1a-mediated effects were emulated by extracellular local application of acid saline solutions (pH = 6.0; HEPES/MES-based solution); and 4) immunofluorescence techniques revealed the presence of ASIC1a antigens on MNTs. These results suggest that ASIC1a channels might be involved in controlling neuromuscular transmission, muscle contraction and fatigue in female mice.

The Concise Guide to PHARMACOLOGY 2013/14: ion channels

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. Ion channels are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Acid-sensing ion channels under hypoxia

Hypoxia represents the lack of oxygen below the basic level, and the range of known channels related to hypoxia is continually increasing. Since abnormal hypoxia initiates pathological processes in numerous diseases via, to a great degree, producing acidic microenvironment, the significance of these channels in this environment has, until now, remained completely unknown. However, recent discovery of acid-sensing ion channels (ASICs) have enhanced our understanding of the hypoxic channelome. They belong to the degenerin/epithelial Na (+) channel family and function once extracellular pH decreases to a certain level. So does the ratiocination emerge that ASICs participate in many hypoxia-induced pathological processes, including pain, apoptosis, malignancy, which all appear to involve them. Since evidence suggests that activity of ASICs is altered under pathological hypoxia, future studies are needed to deeply explore the relationship between ASICs and hypoxia, which may provide a progressive understanding of hypoxic effects in cancer, arthritis, intervertebral disc degeneration, ischemic brain injury and so on.

Neurosensory mechanotransduction through acid-sensing ion channels

Acid-sensing ion channels (ASICs) are voltage-insensitive cation channels responding to extracellular acidification. ASIC proteins have two transmembrane domains and a large extracellular domain. The molecular topology of ASICs is similar to that of the mechanosensory abnormality 4- or 10-proteins expressed in touch receptor neurons and involved in neurosensory mechanotransduction in nematodes. The ASIC proteins are involved in neurosensory mechanotransduction in mammals. The ASIC isoforms are expressed in Merkel cell-neurite complexes, periodontal Ruffini endings and specialized nerve terminals of skin and muscle spindles, so they might participate in mechanosensation. In knockout mouse models, lacking an ASIC isoform produces defects in neurosensory mechanotransduction of tissue such as skin, stomach, colon, aortic arch, venoatrial junction and cochlea. The ASICs are thus implicated in touch, pain, digestive function, baroreception, blood volume control and hearing. However, the role of ASICs in mechanotransduction is still controversial, because we lack evidence that the channels are mechanically sensitive when expressed in heterologous cells. Thus, ASIC channels alone are not sufficient to reconstruct the path of transducing molecules of mechanically activated channels. The mechanotransducers associated with ASICs need further elucidation. In this review, we discuss the expression of ASICs in sensory afferents of mechanoreceptors, findings of knockout studies, technical issues concerning studies of neurosensory mechanotransduction and possible missing links. Also we propose a molecular model and a new approach to disclose the molecular mechanism underlying the neurosensory mechanotransduction.

Acid-sensing ion channels (ASICs) in mouse skeletal muscle afferents are heteromers composed of ASIC1a, ASIC2, and ASIC3 subunits

Acid-sensing ion channels (ASICs) are expressed in skeletal muscle afferents, in which they sense extracellular acidosis and other metabolites released during ischemia and exercise. ASICs are formed as homotrimers or heterotrimers of several isoforms (ASIC1a, ASIC1b, ASIC2a, ASIC2b, and ASIC3), with each channel displaying distinct properties. To dissect the ASIC composition in muscle afferents, we used whole-cell patch-clamp recordings to study the properties of acid-evoked currents (amplitude, pH sensitivity, the kinetics of desensitization and recovery from desensitization, and pharmacological modulation) in isolated, labeled mouse muscle afferents from wild-type (C57BL/6J) and specific ASIC(-/-) mice. We found that ASIC-like currents in wild-type muscle afferents displayed fast desensitization, indicating that they are carried by heteromeric channels. Currents from ASIC1a(-/-) muscle afferents were less pH-sensitive and displayed faster recovery, currents from ASIC2(-/-) mice showed diminished potentiation by zinc, and currents from ASIC3(-/-) mice displayed slower desensitization than those from wild-type mice. Finally, ASIC-like currents were absent from triple-null mice lacking ASIC1a, ASIC2a, and ASIC3. We conclude that ASIC1a, ASIC2a, and ASIC3 heteromers are the principle channels in skeletal muscle afferents. These results will help us understand the role of ASICs in exercise physiology and provide a molecular target for potential drug therapies to treat muscle pain.

Subtype-specific modulation of acid-sensing ion channel (ASIC) function by 2-guanidine-4-methylquinazoline

Acid-sensing ion channels (ASICs) are neuronal Na(+)-selective channels that are transiently activated by extracellular acidification. ASICs are involved in fear and anxiety, learning, neurodegeneration after ischemic stroke, and pain sensation. The small molecule 2-guanidine-4-methylquinazoline (GMQ) was recently shown to open ASIC3 at physiological pH. We have investigated the mechanisms underlying this effect and the possibility that GMQ may alter the function of other ASICs besides ASIC3. GMQ shifts the pH dependence of activation to more acidic pH in ASIC1a and ASIC1b, whereas in ASIC3 this shift goes in the opposite direction and is accompanied by a decrease in its steepness. GMQ also induces an acidic shift of the pH dependence of inactivation of ASIC1a, -1b, -2a, and -3. As a consequence, the activation and inactivation curves of ASIC3 but not other ASICs overlap in the presence of GMQ at pH 7.4, thereby creating a window current. At concentrations >1 mM, GMQ decreases maximal peak currents by reducing the unitary current amplitude. Mutation of residue Glu-79 in the palm domain of ASIC3, previously shown to be critical for channel opening by GMQ, disrupted the GMQ effects on inactivation but not activation. This suggests that this residue is involved in the consequences of GMQ binding rather than in the binding interaction itself. This study describes the mechanisms underlying the effects of a novel class of ligands that modulate the function of all ASICs as well as activate ASIC3 at physiological pH.

How we feel: ion channel partnerships that detect mechanical inputs and give rise to touch and pain perception

Every moment of every day, our skin and its embedded sensory neurons are bombarded with mechanical cues that we experience as pleasant or painful. Knowing the difference between innocuous and noxious mechanical stimuli is critical for survival and relies on the function of mechanoreceptor neurons that vary in their size, shape, and sensitivity. Their function is poorly understood at the molecular level. This review emphasizes the importance of integrating analysis at the molecular and cellular levels and focuses on the discovery of ion channel proteins coexpressed in the mechanoreceptors of worms, flies, and mice.

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.

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.

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.

Effect of a temperature increase in the non-noxious range on proton-evoked ASIC and TRPV1 activity

Acid-sensing ion channels (ASICs) are neuronal H(+)-gated cation channels, and the transient receptor potential vanilloid 1 channel (TRPV1) is a multimodal cation channel activated by low pH, noxious heat, capsaicin, and voltage. ASICs and TRPV1 are present in sensory neurons. It has been shown that raising the temperature increases TRPV1 and decreases ASIC H(+)-gated current amplitudes. To understand the underlying mechanisms, we have analyzed ASIC and TRPV1 function in a recombinant expression system and in dorsal root ganglion (DRG) neurons at room and physiological temperature. We show that temperature in the range studied does not affect the pH dependence of ASIC and TRPV1 activation. A temperature increase induces, however, a small alkaline shift of the pH dependence of steady-state inactivation of ASIC1a, ASIC1b, and ASIC2a. The decrease in ASIC peak current amplitudes at higher temperatures is likely in part due to the observed accelerated open channel inactivation kinetics and for some ASIC types to the changed pH dependence of steady-state inactivation. The increase in H(+)-activated TRPV1 current at the higher temperature is at least in part due to a hyperpolarizing shift in its voltage dependence. The contribution of TRPV1 relative to ASICs to H(+)-gated currents in DRG neurons increases with higher temperature and acidity. Still, ASICs remain the principal pH sensors of DRG neurons at 35°C in the pH range ≥6.

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.

Paeoniflorin, a potent natural compound, protects PC12 cells from MPP+ and acidic damage via autophagic pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Paeoniflorin (PF) is the principal bioactive component of Radix Paeoniae alba, which is widely used in Traditional Chinese Medicine for the treatment of neurodegenerative disorders such as Parkinson's disease (PD).

AIM OF THE STUDY

To evaluate the neuroprotective effects of PF on MPP(+)- or acid- (pH 5.0) induced injury in cultured PC12 cells and to investigate the activity of autophagy-lysosome pathway (ALP). Amiloride (Ami), a non-selective blocker of acid-sensing ion channels (ASICs), as a positive control drug, since it is neuroprotective in rodent models of PD.

MATERIALS AND METHODS

The cell viability was analyzed with MTT assay. The cell injury was assessed by lactate dehydrogenase (LDH) assay. Flow cytometry and Western blot analysis were used to study the apoptotic, calcium influx and autophagic mechanisms.

RESULTS

Ami (100 microM) and PF (50 microM) both protected PC12 cells against MPP(+)- or acid-induced injury as assessed by MTT assay, lactate dehydrogenase release, and apoptosis rate. The concentrations of cytosolic free Ca(2+) were raised after exposure to MPP(+) or acidosis, while Ami and PF both reduced the influx of Ca(2+). More importantly, we found that the mechanisms of neuroprotective effects of Ami and PF were closely associated with the upregulation of LC3-II protein, which is specifically associated with autophagic vacuole membranes. Furthermore, application of MPP(+) or acid induced the overexpression of LAMP2a, which is directly correlated with the activity of the chaperone-mediated autophagy pathway. However, Ami and PF inhibited the overexpression of LAMP2a.

CONCLUSIONS

Our data provide the first experimental evidence that PF modulates autophagy in models of neuron injury, as well as providing the first indication of a relationship between ASICs and ALP.

Acid-sensing ion channels in neurones of the rat suprachiasmatic nucleus

We used reduced slice reparations to study ASIC-like currents in the rat central clock suprachiasmatic nucleus (SCN). In reduced SCN preparations, a drop of extracellular pH evoked a desensitizing inward current to excite SCN neurones to fire at higher rates. Under voltage-clamped conditions, all SCN neurones responded to a 5 s pH step to 6.4 with an inward current that decayed with an average time constant of 1.2 s to 10% of the peak at the end of step. The current was blocked by amiloride with an IC(50) of 14 microm and was carried mainly by Na(+), suggesting an origin of ASIC-like channels. The SCN neurones were sensitive to neutral pH, with 94% of cells responding to pH 7.0 with an inward current. The study of sensitivity to pH between 7.0 and 4.4 revealed a two-component dose-dependent H(+) activation in most SCN neurones, with the first component (85% in amplitude) having a pH(50) of 6.6, and the second (15%) a pH(50) of 5. The ASIC-like currents were potentiated by lactate and low Ca(2+), but were inhibited by Zn(2+). RT-PCR analysis demonstrated the presence of mRNA for ASIC1a, 2a, 2b, and 3 in SCN. Compared to other central neurones, the unique presence of ASIC3 along with ASIC1a in SCN neurones may contribute to the high pH sensitivity and unusual inhibition by Zn(2+). The high pH sensitivity suggests that the SCN neurones are susceptive to extracellular acidification of physiological origins and that the ASIC current might play a role in regulating SCN excitability.