Coupling of proton binding in extracellular domain to channel gating in acid-sensing ion channel

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.

Carotid body chemoreceptors: physiology, pathology, and implications for health and disease

The carotid body (CB) is the main peripheral chemoreceptor for arterial respiratory gases O2 and CO2 and pH, eliciting reflex ventilatory, cardiovascular, and humoral responses to maintain homeostasis. This review examines the fundamental biology underlying CB chemoreceptor function, its contribution to integrated physiological responses, and its role in maintaining health and potentiating disease. Emphasis is placed on 1) transduction mechanisms in chemoreceptor (type I) cells, highlighting the role played by the hypoxic inhibition of O2-dependent K+ channels and mitochondrial oxidative metabolism, and their modification by intracellular molecules and other ion channels; 2) synaptic mechanisms linking type I cells and petrosal nerve terminals, focusing on the role played by the main proposed transmitters and modulatory gases, and the participation of glial cells in regulation of the chemosensory process; 3) integrated reflex responses to CB activation, emphasizing that the responses differ dramatically depending on the nature of the physiological, pathological, or environmental challenges, and the interactions of the chemoreceptor reflex with other reflexes in optimizing oxygen delivery to the tissues; and 4) the contribution of enhanced CB chemosensory discharge to autonomic and cardiorespiratory pathophysiology in obstructive sleep apnea, congestive heart failure, resistant hypertension, and metabolic diseases and how modulation of enhanced CB reactivity in disease conditions may attenuate pathophysiology.

Modulation of acid-sensing ion channels by hydrogen sulfide

Acid-sensing ion channels (ASICs) have been implicated in many physiological and patho-physiological processes like synaptic plasticity, inflammation, pain perception, stroke-induced brain damage and, drug-seeking behaviour. Although ASICs have been shown to be modulated by gasotransmitters like nitric oxide (NO), their regulation by hydrogen sulfide (H₂S) is not known. Here, we present strong evidence that H₂S potentiates ASICs-mediated currents. Low pH-induced current in Chinese hamster ovary (CHO) cells, expressing homomeric either ASIC1a, ASIC2a or ASIC3, increased significantly by an H₂S donor NaHS. The effect was reversed by washing the cells with NaHS-free external solution of pH 7.4. MTSES, a membrane impermeable cysteine thiol-modifier failed to abrogate the effect of NaHS on ASIC1a, suggesting that the target cysteine residues are not in the extracellular region of the channel. The effect of NaHS is not mediated through NO, as the basal NO level in cells did not change following NaHS application. This previously unknown mechanism of ASICs-modulation by H₂S adds a new dimension to the ASICs in health and disease.

Mapping allosteric linkage to channel gating by extracellular domains in the human epithelial sodium channel

The epithelial sodium channel (ENaC) mediates sodium absorption in lung, kidney, and colon epithelia. Channels in the ENaC/degenerin family possess an extracellular region that senses physicochemical changes in the extracellular milieu and allosterically regulates the channel opening. Proteolytic cleavage activates the ENaC opening, by the removal of specific segments in the finger domains of the α- and γ ENaC-subunits. Cleavage causes perturbations in the extracellular region that propagate to the channel gate. However, it is not known how the channel structure mediates the propagation of activation signals through the extracellular sensing domains. Here, to identify the structure-function determinants that mediate allosteric ENaC activation, we performed MD simulations, thiol modification of residues substituted by cysteine, and voltage-clamp electrophysiology recordings. Our simulations of an ENaC heterotetramer, α₁βα₂γ, in the proteolytically cleaved and uncleaved states revealed structural pathways in the α-subunit that are responsible for ENaC proteolytic activation. To validate these findings, we performed site-directed mutagenesis to introduce cysteine substitutions in the extracellular domains of the α-, β-, and γ ENaC-subunits. Insertion of a cysteine at the α-subunit Glu⁵⁵⁷ site, predicted to stabilize a closed state of ENaC, inhibited ENaC basal activity and retarded the kinetics of proteolytic activation by 2-fold. Our results suggest that the lower palm domain of αENaC is essential for ENaC activation. In conclusion, our integrated computational and experimental approach suggests key structure-function determinants for ENaC proteolytic activation and points toward a mechanistic model for the allosteric communication in the extracellular domains of the ENaC/degenerin family channels.

Quercetin inhibits acid-sensing ion channels through a putative binding site in the central vestibular region

Acid-sensing ion channels (ASICs) are associated with many pathophysiological processes, such as neuronal death during ischemic stroke, epileptic seizure and nociception. However, there is a dearth of ASIC-specific therapeutic blockers. Here we report that quercetin, a plant flavonoid, which is known for its neuroprotective effect, reversibly inhibits homomeric rat ASIC1a, ASIC2a and ASIC3 with an IC₅₀ of about 2µM. Also, quercetin prevents low pH-induced intracellular calcium rise and cell death in HEK-293 cells, which have endogenous expression of ASIC1a and 2a. The inhibitory effect of quercetin on ASICs is not due to membrane perturbation, as it did not have any effect on other channels, like NMDA receptor, GABA_A receptor and P2X4 receptor. Unlike quercetin, another flavonoid resveratrol had no effect on ASIC1a. Computational analysis revealed that quercetin binds to the channel in a cavity at the central vestibule, lined by several charged residues like Q276, R369, E373 and E416 in ASIC1a. Mutation of Arg369 to Ala or Glu416 to Gln abolished the inhibitory effect of quercetin on rat ASIC1a completely, while Glu373 to Gln showed reduced sensitivity. Our results raise the possibility of using quercetin for targeting ASICs in vivo.

Novel Insights into Acid-Sensing Ion Channels: Implications for Degenerative Diseases

Degenerative diseases often strike older adults and are characterized by progressive deterioration of cells, eventually leading to tissue and organ degeneration for which limited effective treatment options are currently available. Acid-sensing ion channels (ASICs), a family of extracellular H(+)-activated ligand-gated ion channels, play critical roles in physiological and pathological conditions. Aberrant activation of ASICs is reported to regulate cell apoptosis, differentiation and autophagy. Accumulating evidence has highlighted a dramatic increase and activation of ASICs in degenerative disorders, including multiple sclerosis, Parkinson's disease, Huntington's disease, intervertebral disc degeneration and arthritis. In this review, we have comprehensively discussed the critical roles of ASICs and their potential utility as therapeutic targets in degenerative diseases.

Biophysical properties of acid-sensing ion channels (ASICs)

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

Proton-sensitive cation channels and ion exchangers in ischemic brain injury: new therapeutic targets for stroke?

Ischemic brain injury results from complicated cellular mechanisms. The present therapy for acute ischemic stroke is limited to thrombolysis with the recombinant tissue plasminogen activator (rtPA) and mechanical recanalization. Therefore, a better understanding of ischemic brain injury is needed for the development of more effective therapies. Disruption of ionic homeostasis plays an important role in cell death following cerebral ischemia. Glutamate receptor-mediated ionic imbalance and neurotoxicity have been well established in cerebral ischemia after stroke. However, non-NMDA receptor-dependent mechanisms, involving acid-sensing ion channel 1a (ASIC1a), transient receptor potential melastatin 7 (TRPM7), and Na(+)/H(+) exchanger isoform 1 (NHE1), have recently emerged as important players in the dysregulation of ionic homeostasis in the CNS under ischemic conditions. These H(+)-sensitive channels and/or exchangers are expressed in the majority of cell types of the neurovascular unit. Sustained activation of these proteins causes excessive influx of cations, such as Ca(2+), Na(+), and Zn(2+), and leads to ischemic reperfusion brain injury. In this review, we summarize recent pre-clinical experimental research findings on how these channels/exchangers are regulated in both in vitro and in vivo models of cerebral ischemia. The blockade or transgenic knockdown of these proteins was shown to be neuroprotective in these ischemia models. Taken together, these non-NMDA receptor-dependent mechanisms may serve as novel therapeutic targets for stroke intervention.