Novel Potent Orthosteric Antagonist of ASIC1a Prevents NMDAR-Dependent LTP Induction

A conserved peptide-binding pocket in HyNaC/ASIC ion channels

The only known peptide-gated ion channels-FaNaCs/WaNaCs and HyNaCs-belong to different clades of the DEG/ENaC family. FaNaCs are activated by the short neuropeptide FMRFamide, and HyNaCs by Hydra RFamides, which are not evolutionarily related to FMRFamide. The FMRFamide-binding site in FaNaCs was recently identified in a cleft atop the large extracellular domain. However, this cleft is not conserved in HyNaCs. Here, we combined molecular modeling and site-directed mutagenesis and identified a putative binding pocket for Hydra-RFamides in the extracellular domain of the heterotrimeric HyNaC2/3/5. This pocket localizes to only one of the three subunit interfaces, indicating that this trimeric ion channel binds a single peptide ligand. We engineered an unnatural amino acid at the putative binding pocket entrance, which allowed covalent tethering of Hydra RFamide to the channel, thereby trapping the channel in an open conformation. The identified pocket localizes to the same region as the acidic pocket of acid-sensing ion channels (ASICs), which binds peptide ligands. The pocket in HyNaCs is less acidic, and both electrostatic and hydrophobic interactions contribute to peptide binding. Collectively, our results reveal a conserved ligand-binding pocket in HyNaCs and ASICs and indicate independent evolution of peptide-binding cavities in the two subgroups of peptide-gated ion channels.

On the optical response of novel coumarin-fused NIR BODIPY dyes to X-rays

This paper reports synthesis and characterization of three new coumarin-fused NIR BODIPY dyes 16-18, as well as the detailed study of their optical response to exposure with X-rays (up to 1000 Gy) in solvents of various nature. A strong reaction to irradiation (both in terms of absorption and fluorescence changing) is found in chlorinated solvents (CCl4 and CHCl3) and acetonitrile, while no significant respond of the dyes is observed in toluene and propanol-1. Herewith, their responses turned out to be very versatile: a complex change in fluorescence (quenching of the main band accompanied by the flare-up in a new spectral region) is observed together with colorimetric reaction (e.g., the color of 17 changes from green to blue at 50-80 Gy, and then becomes pink closer to ≈350 Gy). In general, the dyes show good linearity in their response to irradiation up to ≈70-100 Gy and are quite sensitive. For example, the limit of detection (LOD) values for 18 are from 0.29 to 6.73. At the same time, the ratiometric fluorescent response of the compound 16 turns out to be linear over the entire range up to 1000 Gy (to date, this is the first BODIPY-based X-ray probe with optical response over such a wide dose range). Thus, the synthesized dyes seem to be promising for dosimetric support of radiation processing/sterilization procedures.

4-(Azolyl)-Benzamidines as a Novel Chemotype for ASIC1a Inhibitors

Acid-sensing ion channels (ASICs) play a key role in the perception and response to extracellular acidification changes. These proton-gated cation channels are critical for neuronal functions, like learning and memory, fear, mechanosensation and internal adjustments like synaptic plasticity. Moreover, they play a key role in neuronal degeneration, ischemic neuronal injury, seizure termination, pain-sensing, etc. Functional ASICs are homo or heterotrimers formed with (ASIC1-ASIC3) homologous subunits. ASIC1a, a major ASIC isoform in the central nervous system (CNS), possesses an acidic pocket in the extracellular region, which is a key regulator of channel gating. Growing data suggest that ASIC1a channels are a potential therapeutic target for treating a variety of neurological disorders, including stroke, epilepsy and pain. Many studies were aimed at identifying allosteric modulators of ASIC channels. However, the regulation of ASICs remains poorly understood. Using all available crystal structures, which correspond to different functional states of ASIC1, and a molecular dynamics simulation (MD) protocol, we analyzed the process of channel inactivation. Then we applied a molecular docking procedure to predict the protein conformation suitable for the amiloride binding. To confirm the effect of its sole active blocker against the ASIC1 state transition route we studied the complex with another MD simulation run. Further experiments evaluated various compounds in the Enamine library that emerge with a detectable ASIC inhibitory activity. We performed a detailed analysis of the structural basis of ASIC1a inhibition by amiloride, using a combination of in silico approaches to visualize its interaction with the ion pore in the open state. An artificial activation (otherwise, expansion of the central pore) causes a complex modification of the channel structure, namely its transmembrane domain. The output protein conformations were used as a set of docking models, suitable for a high-throughput virtual screening of the Enamine chemical library. The outcome of the virtual screening was confirmed by electrophysiological assays with the best results shown for three hit compounds.

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

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

Acid-sensing ion channel 1a in the central nucleus of the amygdala regulates anxiety-like behaviors in a mouse model of acute pain

Pain is commonly comorbid with anxiety; however, the neural and molecular mechanisms underlying the comorbid anxiety symptoms in pain (CASP) have not been fully elucidated. In this study, we explored the role of acid-sensing ion channel 1a (ASIC1a), located in GABAergic neurons from the central nucleus of the amygdala (GABA^CeA), in the regulation of CASP in an acute pain mouse model. We found that the mice displayed significant mechanical pain sensitization and anxiety-like behaviors one day post injection of complete Freud's adjuvant (CFA1D). Electrophysiological recordings from acute brain slices showed that the activity of GABA^CeA neurons increased in the CFA1D mice compared with that in the saline mice. In addition, chemogenetic inhibition of GABA^CeA neurons relieved mechanical pain sensitization and anxiety-like behaviors in the CFA1D mice. Interestingly, through pharmacological inhibition and genetic knockdown of ASIC1a in the central nucleus amygdala, we found that downregulation of ASIC1a relieved the hypersensitization of mechanical stimuli and alleviated anxiety-related behaviors, accompanied with reversing the hyperactivity of GABA^CeA neurons in the CFA 1D mice. In conclusion, our results provide novel insights that ASIC1a in GABA^CeA neurons regulates anxiety-like behaviors in a mouse model of acute pain.

Mechanisms of Action of the Peptide Toxins Targeting Human and Rodent Acid-Sensing Ion Channels and Relevance to Their In Vivo Analgesic Effects

Acid-sensing ion channels (ASICs) are voltage-independent H⁺-gated cation channels largely expressed in the nervous system of rodents and humans. At least six isoforms (ASIC1a, 1b, 2a, 2b, 3 and 4) associate into homotrimers or heterotrimers to form functional channels with highly pH-dependent gating properties. This review provides an update on the pharmacological profiles of animal peptide toxins targeting ASICs, including PcTx1 from tarantula and related spider toxins, APETx2 and APETx-like peptides from sea anemone, and mambalgin from snake, as well as the dimeric protein snake toxin MitTx that have all been instrumental to understanding the structure and the pH-dependent gating of rodent and human cloned ASICs and to study the physiological and pathological roles of native ASICs in vitro and in vivo. ASICs are expressed all along the pain pathways and the pharmacological data clearly support a role for these channels in pain. ASIC-targeting peptide toxins interfere with ASIC gating by complex and pH-dependent mechanisms sometimes leading to opposite effects. However, these dual pH-dependent effects of ASIC-inhibiting toxins (PcTx1, mambalgin and APETx2) are fully compatible with, and even support, their analgesic effects in vivo, both in the central and the peripheral nervous system, as well as potential effects in humans.

Pharmacological Validation of ASIC1a as a Druggable Target for Neuroprotection in Cerebral Ischemia Using an Intravenously Available Small Molecule Inhibitor

Acidosis is a hallmark of ischemic stroke and a promising neuroprotective target for preventing neuronal injury. Previously, genetic manipulations showed that blockade of acid-sensing ion channel 1a (ASIC1a)-mediated acidotoxicity could dramatically alleviate the volume of brain infarct and restore neurological function after cerebral ischemia. However, few pharmacological candidates have been identified to exhibit efficacy on ischemic stroke through inhibition of ASIC1a. In this work, we examined the ability of a toxin-inspired compound 5b (C5b), previously found to effectively inhibit ASIC1a in vitro, to exert protective effects in animal models of ischemic stroke in vivo. We found that C5b exerts significant neuroprotective effects not only in acid-induced neuronal death in vitro but also ischemic brain injury in vivo, suggesting that ASIC1a is a druggable target for therapeutic development. More importantly, C5b is able to cross the blood brain barrier and significantly reduce brain infarct volume when administered intravenously in the ischemic animal model, highlighting its systemic availability for therapies against neurodegeneration due to acidotoxicity. Together, our data demonstrate that C5b is a promising lead compound for neuroprotection through inhibiting ASIC1a, which warrants further translational studies.

Neurodegenerative Disease: What Potential Therapeutic Role of Acid-Sensing Ion Channels?

Acidic pH shift occurs in many physiological neuronal activities such as synaptic transmission and synaptic plasticity but also represents a characteristic feature of many pathological conditions including inflammation and ischemia. Neuroinflammation is a complex process that occurs in various neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, and Huntington’s disease. Acid-sensing ion channels (ASICs) represent a widely expressed pH sensor in the brain that play a key role in neuroinflammation. On this basis, acid-sensing ion channel blockers are able to exert neuroprotective effects in different neurodegenerative diseases. In this review, we discuss the multifaceted roles of ASICs in brain physiology and pathology and highlight ASIC1a as a potential pharmacological target in neurodegenerative diseases.

Molecular mechanism and structural basis of small-molecule modulation of the gating of acid-sensing ion channel 1

Acid-sensing ion channels (ASICs) are proton-gated cation channels critical for neuronal functions. Studies of ASIC1, a major ASIC isoform and proton sensor, have identified acidic pocket, an extracellular region enriched in acidic residues, as a key participant in channel gating. While binding to this region by the venom peptide psalmotoxin modulates channel gating, molecular and structural mechanisms of ASIC gating modulation by small molecules are poorly understood. Here, combining functional, crystallographic, computational and mutational approaches, we show that two structurally distinct small molecules potently and allosterically inhibit channel activation and desensitization by binding at the acidic pocket and stabilizing the closed state of rat/chicken ASIC1. Our work identifies a previously unidentified binding site, elucidates a molecular mechanism of small molecule modulation of ASIC gating, and demonstrates directly the structural basis of such modulation, providing mechanistic and structural insight into ASIC gating, modulation and therapeutic targeting.

Histamine and Corticosterone Modulate Acid Sensing Ion Channels (ASICs) Dependent Long-term Potentiation at the Mouse Anterior Cingulate Cortex

Increase in proton concentration [H⁺] or decrease in local and global extracellular pH occurs in both physiological and pathological conditions. Acid-sensing ion channels (ASICs), belonging to the ENaC/Deg superfamily, play an important role in signal transduction as proton sensor. ASICs and in particular ASIC1a (one of the six ASICs subunits) which is permeable to Ca²⁺, are involved in many physiological processes including synaptic plasticity and neurodegenerative diseases. Activity-dependent long-term potentiation (LTP) is a major type of long-lasting synaptic plasticity in the CNS, associated with learning, memory, development, fear and persistent pain. Neurons in the anterior cingulate cortex (ACC) play critical roles in pain perception and chronic pain and express ASIC1a channels. During synaptic transmission, acidification of the synaptic cleft presumably due to the co-release of neurotransmitter and H⁺ from synaptic vesicles activates postsynaptic ASIC1a channels in ACC of mice. This generates ASIC1a synaptic currents that add to the glutamatergic excitatory postsynaptic currents (EPSCs). Here we report that modulators like histamine and corticosterone, acting through ASIC1a regulate synaptic plasticity, reducing the threshold for LTP induction of glutamatergic EPSCs. Our findings suggest a new role for ASIC1a mediating the neuromodulator action of histamine and corticosterone regulating specific forms of synaptic plasticity in the mouse ACC.

Distribution of Acid Sensing Ion Channels in Axonal Growth Cones and Presynaptic Membrane of Cultured Hippocampal Neurons

Although acid-sensing ion channels (ASICs) are widely expressed in the central nervous system, their distribution and roles in axonal growth cones remain unclear. In this study, we examined ASIC localization and function in the axonal growth cones of cultured immature hippocampal neurons. Our immunocytochemical data showed that native and overexpressed ASIC1a and ASIC2a are both localized in growth cones of cultured young hippocampal neurons. Calcium imaging and electrophysiological assay results were utilized to validate their function. The calcium imaging test results indicated that the ASICs (primarily ASIC1a) present in growth cones mediate calcium influx despite the addition of voltage-gated Ca²⁺ channels antagonists and the depletion of intracellular calcium stores. The electrophysiological tests results suggested that a rapid decrease in extracellular pH at the growth cones of voltage-clamped neurons elicits inward currents that were blocked by bath application of the ASIC antagonist amiloride, showing that the ASICs expressed at growth cones are functional. The subsequent immuno-colocalization test results demonstrated that ASIC1a and ASIC2a are both colocalized with Neurofilament-H and Bassoon in mature hippocampal neurons. This finding demonstrated that after reaching maturity, ASIC1a and ASIC2a are both distributed in axons and the presynaptic membrane. Our data reveal the distribution of functional ASICs in growth cones of immature hippocampal neurons and the presence of ASICs in the axons and presynaptic membrane of mature hippocampal neurons, indicating a possible role for ASICs in axonal guidance, synapse formation and neurotransmitter release.

Bilirubin enhances the activity of ASIC channels to exacerbate neurotoxicity in neonatal hyperbilirubinemia in mice

Neonatal hyperbilirubinemia is a common clinical condition that can lead to brain encephalopathy, particularly when concurrent with acidosis due to infection, ischemia, and hypoxia. The prevailing view is that acidosis increases the permeability of the blood-brain barrier to bilirubin and exacerbates its neurotoxicity. In this study, we found that the concentration of the cell death marker, lactate dehydrogenase (LDH) in cerebrospinal fluid (CSF), is elevated in infants with both hyperbilirubinemia and acidosis and showed stronger correlation with the severity of acidosis rather than increased bilirubin concentration. In mouse neonatal neurons, bilirubin exhibits limited toxicity but robustly potentiates the activity of acid-sensing ion channels (ASICs), resulting in increases in intracellular Ca2+ concentration, spike firings, and cell death. Furthermore, neonatal conditioning with concurrent hyperbilirubinemia and hypoxia-induced acidosis promoted long-term impairments in learning and memory and complex sensorimotor functions in vivo, which are largely attenuated in ASIC1a null mice. These findings suggest that targeting acidosis and ASICs may attenuate neonatal hyperbilirubinemia complications.

Role of ASIC1a in Normal and Pathological Synaptic Plasticity

Acid-sensing ion channels (ASICs), members of the degenerin/epithelial Na+ channel superfamily, are broadly distributed in the mammalian nervous system where they play important roles in a variety of physiological processes, including neurotransmission and memory-related behaviors. In the last few years, we and others have investigated the role of ASIC1a in different forms of synaptic plasticity especially in the CA1 area of the hippocampus. This review summarizes the latest research linking ASIC1a to synaptic function either in physiological or pathological conditions. A better understanding of how these channels are regulated in brain circuitries relevant to synaptic plasticity and memory may offer novel targets for pharmacological intervention in neuropsychiatric and neurological disorders.

Synaptic signals mediated by protons and acid-sensing ion channels

Extracellular pH changes may constitute significant signals for neuronal communication. During synaptic transmission, changes in pH in the synaptic cleft take place. Its role in the regulation of presynaptic Ca²⁺ currents through multivesicular release in ribbon-type synapses is a proven phenomenon. In recent years, protons have been recognized as neurotransmitters that participate in neuronal communication in synapses of several regions of the CNS such as amygdala, nucleus accumbens, and brainstem. Protons are released by nerve stimulation and activate postsynaptic acid-sensing ion channels (ASICs). Several types of ASIC channels are expressed in the peripheral and central nervous system. The influx of Ca²⁺ through some subtypes of ASICs, as a result of synaptic transmission, agrees with the participation of ASICs in synaptic plasticity. Pharmacological and genetical inhibition of ASIC1a results in alterations in learning, memory, and phenomena like fear and cocaine-seeking behavior. The recognition of endogenous molecules, such as arachidonic acid, cytokines, histamine, spermine, lactate, and neuropeptides, capable of inhibiting or potentiating ASICs suggests the existence of mechanisms of synaptic modulation that have not yet been fully identified and that could be tuned by new emerging pharmacological compounds with potential therapeutic benefits.

Acid-Sensing Ion Channel 1a Is Involved in N-Methyl D-Aspartate Receptor-Dependent Long-Term Depression in the Hippocampus

Acid-sensing ion channels (ASICs), members of the degenerin/epithelial Na⁺ channel superfamily, are largely expressed in the mammalian nervous system. ASIC1a is highly permeable to Ca²⁺ and are involved in many physiological processes, including synaptic plasticity, learning, and memory. To clarify the role of ASIC1a in synaptic transmission and plasticity, we investigated N-methyl D-aspartate (NMDA) receptor-dependent long-term depression (LTD) in the CA1 region of the hippocampus. We found that: (1) ASIC1a mediates a component of ASIC1a excitatory postsynaptic currents (EPSCs); (2) ASIC1a plays a role in electrical LTD induced by LFS protocol both in P13-18 and P30-40 animals; (3) ASIC1a is involved in chemical LTD induced by brief bath application of NMDA both in P13-18 and P30-40 animals; and finally (4) a functional interaction between ASIC1a and NMDA receptors occurs during LTD. These findings suggest a new role for ASIC1a in specific forms of synaptic plasticity in the mouse hippocampus.

Protein Kinase C Lambda Mediates Acid-Sensing Ion Channel 1a-Dependent Cortical Synaptic Plasticity and Pain Hypersensitivity

Chronic pain is a serious debilitating disease for which effective treatment is still lacking. Acid-sensing ion channel 1a (ASIC1a) has been implicated in nociceptive processing at both peripheral and spinal neurons. However, whether ASIC1a also contributes to pain perception at the supraspinal level remains elusive. Here, we report that ASIC1a in ACC is required for thermal and mechanical hypersensitivity associated with chronic pain. ACC-specific genetic deletion or pharmacological blockade of ASIC1a reduced the probability of cortical LTP induction and attenuated inflammatory thermal hyperalgesia and mechanical allodynia in male mice. Using cell type-specific manipulations, we demonstrate that ASIC1a in excitatory neurons of ACC is a major player in cortical LTP and pain behavior. Mechanistically, we show that ASIC1a tuned pain-related cortical plasticity through protein kinase C λ-mediated increase of membrane trafficking of AMPAR subunit GluA1 in ACC. Importantly, postapplication of ASIC1a inhibitors in ACC reversed previously established nociceptive hypersensitivity in both chronic inflammatory pain and neuropathic pain models. These results suggest that ASIC1a critically contributes to a higher level of pain processing through synaptic potentiation in ACC, which may serve as a promising analgesic target for treatment of chronic pain.SIGNIFICANCE STATEMENT Chronic pain is a debilitating disease that still lacks effective therapy. Ion channels are good candidates for developing new analgesics. Here, we provide several lines of evidence to support an important role of cortically located ASIC1a channel in pain hypersensitivity through promoting long-term synaptic potentiation in the ACC. Our results indicate a promising translational potential of targeting ASIC1a to treat chronic pain.

The acid-sensing ion channel ASIC1a mediates striatal synapse remodeling and procedural motor learning

Acid-sensing ion channel 1a (ASIC1a) is abundant in multiple brain regions, including the striatum, which serves as the input nucleus of the basal ganglia and is critically involved in procedural learning and motor memory. We investigated the functional role of ASIC1a in striatal neurons. We found that ASIC1a was critical for striatum-dependent motor coordination and procedural learning by regulating the synaptic plasticity of striatal medium spiny neurons. Global deletion of Asic1a in mice led to increased dendritic spine density but impaired spine morphology and postsynaptic architecture, which were accompanied by the decreased function of N-methyl-d-aspartate (NMDA) receptors at excitatory synapses. These structural and functional changes caused by the loss of ASIC1a were largely mediated by reduced activation (phosphorylation) of Ca2+/calmodulin-dependent protein kinase II (CaMKII) and extracellular signal-regulated protein kinases (ERKs). Consequently, Asic1a null mice exhibited poor performance on multiple motor tasks, which was rescued by striatal-specific expression of either ASIC1a or CaMKII. Together, our data reveal a previously unknown mechanism mediated by ASIC1a that promotes the excitatory synaptic function underlying striatum-related procedural learning and memory.

Subtype-selective inhibition of acid-sensing ion channel 3 by a natural flavonoid

AIMS

Acid-sensing ion channels (ASICs) are extracellular proton-gated cation channels that have been implicated in multiple physiological and pathological processes, and peripheral ASIC3 prominently participate into the pathogenesis of chronic pain, itch, and neuroinflammation, which necessitates the need for discovery and development of novel modulators in a subtype-specific manner.

METHODS

Whole-cell patch clamp recordings and behavioral assays were used to examine the effect of several natural compounds on the ASIC-mediated currents and acid-induced nocifensive behavior, respectively.

RESULTS

We identified a natural flavonoid compound, (-)-epigallocatechin gallate (EGCG, compound 11), that acts as a potent inhibitor for the ASIC3 channel in an isoform-specific way. The compound 11 inhibited ASIC3 currents with an apparent half maximal inhibitory concentration of 13.2 μmol/L when measured at pH 5.0. However, at the concentration up to 100 μmol/L, the compound 11 had no significant impacts on the homomeric ASIC1a, 1b, and 2a channels. In contrast to most of the known ASIC inhibitors that usually bear either basic or carboxylic groups, the compound 11 belongs to the polyphenolic family. In compound 11, both the chirality and the 3-hydroxyl group of its pyrogallol part, in addition to the integrity of the gallate part, are crucial for the inhibitory efficacy. Finally, EGCG was found significantly to decrease the acid-induced nocifensive behavior in mice.

CONCLUSION

Taken together, these results thus defined a novel backbone structure for small molecule drug design targeting ASIC3 channels to treat pain-related diseases.

Novel amidrazone derivatives: Design, synthesis and activity evaluation

A series of new 6-styryl-naphthalene-2-amidrazone derivatives were synthesized and evaluated as potential ASIC1a inhibitors. Among them, compound 5e showed the most activity to inhibit [Ca²⁺]_i. elevation in acid-induced articular chondrocytes. Together with the important role of ASIC1a in the pathogenesis of tissue acidification diseases including rheumatoid arthritis, these results might provide a meaningful hint or inspiration in developing drugs targeting at tissue acidification diseases.

Computational elucidation, mutational and hot spot-based designing of potential inhibitors against human acid-sensing ion channels (hASIC-1a) to treat various physiological conditions

Acid-sensing ion channels are ligand/proton-gated ion channels belonging to the family of the degenerin/epithelial Na⁺ channel (DEG/ENaC). They function as a sodium-selective pore for Ca²⁺ entry into neuronal cells during pathological conditions. The blocking of this channel has therapeutic importance, because at basal physiological pH (7.2), it is in a closed state and under a more acidic condition, and the ASIC1a ion channel is activated. To investigate the different states of the hASIC1a channel based on mutational analysis, structure-based virtual screening and molecular dynamics simulation studies. The system showed stability after 30 ns (after 1500 frame), and it was stabilized to an average value around 2.2Å. During the simulation, the ion channel residues in persistent contact with toxin PcTx1 were D237, E238, D347, D351, E219 and E355. These residues are important physiologically for the activation of the channel. From in silico alanine scanning, the significant hotspots obtained in hASIC1 are E344, P347, F352, D351, E355 and E219. From the sitemap analysis, it was evident that the sitemap found one of the active sites at the PcTx1 binding site with a site score of 1.086 and a D-score of 1.035 for hASIC1. We obtained a few promising hits and final potential hits from the virtual screening in hASIC1 that made interactions with the residues in the acidic pocket (E344, P347, F352, D351, E355 and E219). Based on these studies, the hits and scaffolds of potential therapeutic interest against various pathological conditions are associated with hASIC1a for future studies.

Acid-sensing ion channels regulate spontaneous inhibitory activity in the hippocampus: possible implications for epilepsy

Acid-sensing ion channels (ASICs) play an important role in numerous functions in the central and peripheral nervous systems ranging from memory and emotions to pain. The data correspond to a recent notion that each neuron and many glial cells of the mammalian brain express at least one member of the ASIC family. However, the mechanisms underlying the involvement of ASICs in neuronal activity are poorly understood. However, there are two exceptions, namely, the straightforward role of ASICs in proton-based synaptic transmission in certain brain areas and the role of the Ca(2+)-permeable ASIC1a subtype in ischaemic cell death. Using a novel orthosteric ASIC antagonist, we have found that ASICs specifically control the frequency of spontaneous inhibitory synaptic activity in the hippocampus. Inhibition of ASICs leads to a strong increase in the frequency of spontaneous inhibitory postsynaptic currents. This effect is presynaptic because it is fully reproducible in single synaptic boutons attached to isolated hippocampal neurons. In concert with this observation, inhibition of the ASIC current diminishes epileptic discharges in a low Mg(2+) model of epilepsy in hippocampal slices and significantly reduces kainate-induced discharges in the hippocampus in vivo Our results reveal a significant novel role for ASICs.This article is part of the themed issue 'Evolution brings Ca(2+) and ATP together to control life and death'.

Potentiation and Block of ASIC1a by Memantine

Acid-sensing ion channels (ASICs) are modulated by various classes of ligands, including the recently described hydrophobic monoamines, which inhibit and potentiate ASICs in a subunit-specific manner. In particular, memantine inhibits ASIC1a and potentiates ASIC2a homomers. The aim of the present work was to characterize action mechanism of memantine on recombinant ASIC1a expressed in CHO (Chinese hamster ovary) cells. We have demonstrated that effect of memantine on ASIC1a strongly depends on membrane voltage, conditioning pH value and application protocol. When applied simultaneously with activating acidification at hyperpolarized voltages, memantine caused the strongest inhibition. Surprisingly, application of memantine between ASIC1a activations at zero voltage caused significant potentiation. Analysis of the data suggests that memantine produces two separate effects, voltage-dependent open-channel block and shift of steady-state desensitization curve to more acidic values. Putative binding sites are discussed based on the computer docking of memantine to the acidic pocket and the pore region.

Xuefu zhuyu decoction improves cognitive impairment in experimental traumatic brain injury via synaptic regulation

An overarching consequence of traumatic brain injury (TBI) is the cognitive impairment. It may hinder individual performance of daily tasks and determine people's subjective well-being. The damage to synaptic plasticity, one of the key mechanisms of cognitive dysfunction, becomes the potential therapeutic strategy of TBI. In this study, we aimed to investigate whether Xuefu Zhuyu Decoction (XFZYD), a traditional Chinese medicine, provided a synaptic regulation to improve cognitive disorder following TBI. Morris water maze and modified neurological severity scores were performed to assess the neurological and cognitive abilities. The PubChem Compound IDs of the major compounds of XFZYD were submitted into BATMAN-TCM, an online bioinformatics analysis tool, to predict the druggable targets related to synaptic function. Furthermore, we validated the prediction through immunohistochemical, RT-PCR and western blot analyses. We found that XFZYD enhanced neuroprotection, simultaneously improved learning and memory performances in controlled cortical impact rats. Bioinformatics analysis revealed that the improvements of XFZYD implied the Long-term potentiation relative proteins including NMDAR1, CaMKII and GAP-43. The further confirmation of molecular biological studies confirmed that XFZYD upregulated the mRNA and protein levels of NMDAR1, CaMKII and GAP-43. Pharmacological synaptic regulation of XFZYD could provide a novel therapeutic strategy for cognitive impairment following TBI.

A modulatory role of ASICs on GABAergic synapses in rat hippocampal cell cultures

Rapid acidification occurring during synaptic vesicle release can activate acid-sensing ion channels (ASICs) both on pre- and postsynaptic neurons. In the latter case, a fraction of postsynaptic current would be mediated by cation-selective acid-sensing ion channels. Additionally, in both cases, activation of acid-sensing ion channels could modulate synaptic strength by affecting transmitter release and/or sensitivity of postsynaptic receptors. To address potential involvement of acid-sensing ion channels in mediation/modulation of synaptic transmission at hippocampal GABAergic synapses, we studied effects of three structurally different blockers of acid-sensing ion channels on evoked postsynaptic currents using the patch-clamp technique. We found that GABAergic postsynaptic currents, recorded below their reversal potential as inward currents, are suppressed by all the employed blockers of acid-sensing ion channels. These currents were suppressed by ~ 20 % in the presence of a novel blocker 5b (1 μM) and by ~30 % in the presence of either amiloride (25 μM) or diminazene (20 μM). In the same cells the suppression of postsynaptic currents, recorded above their reversal potential as outward currents was statistically insignificant. These results imply that the effects of blockers in our experiments are at least partially postsynaptic. On the other hand, in the case of mediation of a fraction of postsynaptic current by acid-sensing ion channels, an increase of outward currents would be expected under our experimental conditions. Our analysis of a bicuculline-resistant fraction of postsynaptic currents also suggests that effects of the blockers are predominantly modulatory. In this work we present evidence for the first time that acid-sensing ion channels play a functional role at hippocampal GABAergic synapses. The suppressing effect of the blockers of acid-sensing ion channels on GABAergic transmission is due, at least partially, to a postsynaptic but (predominantly) modulatory mechanism. We hypothesize that the modulatory effect is due to functional crosstalk between ASICs and GABA_A-receptors recently reported in isolated neurons, however, verification of this hypothesis is necessary.

Histamine selectively potentiates acid-sensing ion channel 1a

Although acid-sensitive ion channels (ASICs) play an important role in brain functions, the exact mechanism of their physiological activation remain unclear. A possible answer to the intriguing question is that some presently unknown endogenous ligand(s) positively modulate ASICs and enhance their responses to physiologically significant level. In the present work we found that histamine selectively potentiates ASIC1a homomers in CHO cells. Action of histamine was particularly pronounced at modest acidifications, which cause minor response. At these conditions micromolar concentrations of histamine have provided significant potentiation of ASIC1a response. We proposed that histamine and possibly some other endogenous amines can positively modulate ASICs functions.

Acid-sensing ion channel 1a contributes to hippocampal LTP inducibility through multiple mechanisms

The exact roles of acid-sensing ion channels (ASICs) in synaptic plasticity remain elusive. Here, we address the contribution of ASIC1a to five forms of synaptic plasticity in the mouse hippocampus using an in vitro multi-electrode array recording system. We found that genetic deletion or pharmacological blockade of ASIC1a greatly reduced, but did not fully abolish, the probability of long-term potentiation (LTP) induction by either single or repeated high frequency stimulation or theta burst stimulation in the CA1 region. However, these treatments did not affect hippocampal long-term depression induced by low frequency electrical stimulation or (RS)-3,5-dihydroxyphenylglycine. We also show that ASIC1a exerts its action in hippocampal LTP through multiple mechanisms that include but are not limited to augmentation of NMDA receptor function. Taken together, these results reveal new insights into the role of ASIC1a in hippocampal synaptic plasticity and the underlying mechanisms. This unbiased study also demonstrates a novel and objective way to assay synaptic plasticity mechanisms in the brain.

Fluorescence-Based Automated Screening Assay for the Study of the pH-Sensitive Channel ASIC1a

Acid-sensing ion channel 1a (ASIC1a) is involved in several pathologies, including neurodegenerative and neuroinflammatory disorders, stroke, epilepsy, and inflammatory pain. ASIC1a has been the subject of intense drug discovery programs devoted to the development of new pharmacological tools for its modulation. However, these efforts to generate new compounds have faced the lack of an efficient screening procedure. In the past decades, improvements in screening technologies and fluorescent sensors for the study of ion channels have provided new opportunities in this field. Unfortunately, ASIC1a is mainly a Na+ permeable channel and undergoes desensitization after its activation, two features that make the use of the available screening procedures problematic. We propose here a novel screening approach for the study of ASIC1a activity in full automation. Our method is based on the stimulation of ASIC1a-expressing cells by protons and the use of electrochromic fluorescent voltage sensors as a readout of ion channel activation. This method will prove to be useful for drug screening programs aimed at ASIC1a modulation.