Recombinant production and solution structure of PcTx1, the specific peptide inhibitor of ASIC1a proton-gated cation channels

Multiple mechanisms of action of an extremely painful venom

Evolutionary arms races between predator and prey can lead to extremely specific and effective defense mechanisms. Such defenses include venoms that deter predators by targeting nociceptive (pain-sensing) pathways. Through co-evolution, venom toxins can become extremely efficient modulators of their molecular targets. The venom of velvet ants (Hymenoptera: Mutillidae) is notoriously painful. The intensity of a velvet ant sting has been described as "Explosive and long lasting, you sound insane as you scream. Hot oil from the deep fryer spilling over your entire hand." [1] The effectiveness of the velvet ant sting as a deterrent against potential predators has been shown across vertebrate orders, including mammals, amphibians, reptiles, and birds [2-4]. The venom's low toxicity suggests it has a targeted effect on nociceptive sensory mechanisms [5]. This leads to the hypothesis that velvet ant venom targets a conserved nociception mechanism, which we sought to uncover using Drosophila melanogaster as a model system. Drosophila larvae have peripheral sensory neurons that sense potentially damaging (noxious) stimuli such as high temperature, harsh mechanical touch, and noxious chemicals [6-9]. These polymodal nociceptors are called class IV multidendritic dendritic arborizing (cIV da) neurons, and they share many features with vertebrate nociceptors, including conserved sensory receptor channels [10,11]. We found that velvet ant venom strongly activated Drosophila nociceptors through heteromeric Pickpocket/Balboa (Ppk/Bba) ion channels. Furthermore, we found a single venom peptide (Do6a) that activated larval nociceptors at nanomolar concentrations through Ppk/Bba. Drosophila Ppk/Bba is homologous to mammalian Acid Sensing Ion Channels (ASICs) [12]. However, the Do6a peptide did not produce behavioral signs of nociception in mice, which was instead triggered by other non-specific, less potent, peptides within the venom. This suggests that Do6a is an insect-specific venom component that potently activates insect nociceptors. Consistent with this, we showed that the velvet ant's defensive sting produced aversive behavior in a predatory praying mantis. Together, our results indicate that velvet ant venom evolved to target nociceptive systems of both vertebrates and invertebrates, but through different molecular mechanisms.

ScrepYard: An online resource for disulfide-stabilized tandem repeat peptides

Receptor avidity through multivalency is a highly sought-after property of ligands. While readily available in nature in the form of bivalent antibodies, this property remains challenging to engineer in synthetic molecules. The discovery of several bivalent venom peptides containing two homologous and independently folded domains (in a tandem repeat arrangement) has provided a unique opportunity to better understand the underpinning design of multivalency in multimeric biomolecules, as well as how naturally occurring multivalent ligands can be identified. In previous work, we classified these molecules as a larger class termed secreted cysteine-rich repeat-proteins (SCREPs). Here, we present an online resource; ScrepYard, designed to assist researchers in identification of SCREP sequences of interest and to aid in characterizing this emerging class of biomolecules. Analysis of sequences within the ScrepYard reveals that two-domain tandem repeats constitute the most abundant SCREP domain architecture, while the interdomain "linker" regions connecting the functional domains are found to be abundant in amino acids with short or polar sidechains and contain an unusually high abundance of proline residues. Finally, we demonstrate the utility of ScrepYard as a virtual screening tool for discovery of putatively multivalent peptides, by using it as a resource to identify a previously uncharacterized serine protease inhibitor and confirm its predicted activity using an enzyme assay.

Recombinant Expression in Pichia pastoris System of Three Potent Kv1.3 Channel Blockers: Vm24, Anuroctoxin, and Ts6

The Kv1.3 channel has become a therapeutic target for the treatment of various diseases. Several Kv1.3 channel blockers have been characterized from scorpion venom; however, extensive studies require amounts of toxin that cannot be readily obtained directly from venoms. The Pichia pastoris expression system provides a cost-effective approach to overcoming the limitations of chemical synthesis and E. coli recombinant expression. In this work, we developed an efficient system for the production of three potent Kv1.3 channel blockers from different scorpion venoms: Vm24, AnTx, and Ts6. Using the Pichia system, these toxins could be obtained in sufficient quantities (Vm24 1.6 mg/L, AnTx 46 mg/L, and Ts6 7.5 mg/L) to characterize their biological activity. A comparison was made between the activity of tagged and untagged recombinant peptides. Tagged Vm24 and untagged AnTx are nearly equivalent to native toxins in blocking Kv1.3 (Kd = 4.4 pM and Kd = 0.72 nM, respectively), whereas untagged Ts6 exhibits a 53-fold increase in Kd (Kd = 29.1 nM) as compared to the native peptide. The approach described here provides a method that can be optimized for toxin production to develop more selective and effective Kv1.3 blockers with therapeutic potential.

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.

Acid-Sensing Ion Channels in Glial Cells

Acid-sensing ion channels (ASICs) are proton-gated cation channels and key mediators of responses to neuronal injury. ASICs exhibit unique patterns of distribution in the brain, with high expression in neurons and low expression in glial cells. While there has been a lot of focus on ASIC in neurons, less is known about the roles of ASICs in glial cells. ASIC1a is expressed in astrocytes and might contribute to synaptic transmission and long-term potentiation. In oligodendrocytes, constitutive activation of ASIC1a participates in demyelinating diseases. ASIC1a, ASIC2a, and ASIC3, found in microglial cells, could mediate the inflammatory response. Under pathological conditions, ASIC dysregulation in glial cells can contribute to disease states. For example, activation of astrocytic ASIC1a may worsen neurodegeneration and glioma staging, activation of microglial ASIC1a and ASIC2a may perpetuate ischemia and inflammation, while oligodendrocytic ASIC1a might be involved in multiple sclerosis. This review concentrates on the unique ASIC components in each of the glial cells and integrates these glial-specific ASICs with their physiological and pathological conditions. Such knowledge provides promising evidence for targeting of ASICs in individual glial cells as a therapeutic strategy for a diverse range of conditions.

Strategies for Heterologous Expression, Synthesis, and Purification of Animal Venom Toxins

Animal venoms are complex mixtures containing peptides and proteins known as toxins, which are responsible for the deleterious effect of envenomations. Across the animal Kingdom, toxin diversity is enormous, and the ability to understand the biochemical mechanisms governing toxicity is not only relevant for the development of better envenomation therapies, but also for exploiting toxin bioactivities for therapeutic or biotechnological purposes. Most of toxinology research has relied on obtaining the toxins from crude venoms; however, some toxins are difficult to obtain because the venomous animal is endangered, does not thrive in captivity, produces only a small amount of venom, is difficult to milk, or only produces low amounts of the toxin of interest. Heterologous expression of toxins enables the production of sufficient amounts to unlock the biotechnological potential of these bioactive proteins. Moreover, heterologous expression ensures homogeneity, avoids cross-contamination with other venom components, and circumvents the use of crude venom. Heterologous expression is also not only restricted to natural toxins, but allows for the design of toxins with special properties or can take advantage of the increasing amount of transcriptomics and genomics data, enabling the expression of dormant toxin genes. The main challenge when producing toxins is obtaining properly folded proteins with a correct disulfide pattern that ensures the activity of the toxin of interest. This review presents the strategies that can be used to express toxins in bacteria, yeast, insect cells, or mammalian cells, as well as synthetic approaches that do not involve cells, such as cell-free biosynthesis and peptide synthesis. This is accompanied by an overview of the main advantages and drawbacks of these different systems for producing toxins, as well as a discussion of the biosafety considerations that need to be made when working with highly bioactive proteins.

Optimization of Pichia pastoris Expression System for High-Level Production of Margatoxin

Margatoxin (MgTx) is a high-affinity blocker of voltage-gated potassium (Kv) channels. It inhibits Kv1.1–Kv1.3 ion channels in picomolar concentrations. This toxin is widely used to study physiological function of Kv ion channels in various cell types, including immune cells. Isolation of native MgTx in large quantities from scorpion venom is not affordable. Chemical synthesis and recombinant production in Escherichia coli need in vitro oxidative refolding for proper disulfide bond formation, resulting in a very low yield of peptide production. The Pichia pastoris expression system offers an economical approach to overcome all these limitations and gives a higher yield of correctly refolded recombinant peptides. In this study, improved heterologous expression of recombinant MgTx (rMgTx) in P. pastoris was obtained by using preferential codons, selecting the hyper-resistant clone against Zeocin, and optimizing the culturing conditions. About 36 ± 4 mg/L of >98% pure His-tagged rMgTx (TrMgTx) was produced, which is a threefold higher yield than has been previously reported. Proteolytic digestion of TrMgTx with factor Xa generated untagged rMgTx (UrMgTx). Both TrMgTx and UrMgTx blocked the Kv1.2 and Kv1.3 currents (patch-clamp) (K_d for Kv1.2 were 64 and 14 pM, and for Kv1.3, 86 and 50 pM, respectively) with comparable potency to the native MgTx. The analysis of the binding kinetics showed that TrMgTx had a lower association rate than UrMgTx for both Kv1.2 and Kv1.3. The dissociation rate of both the analogues was the same for Kv1.3. However, in the case of Kv1.2, TrMgTx showed a much higher dissociation rate with full recovery of the block than UrMgTx. Moreover, in a biological functional assay, both peptides significantly downregulated the expression of early activation markers IL2R and CD40L in activated CD4⁺ T_EM lymphocytes whose activation was Kv1.3 dependent. In conclusion, the authors report that the Pichia expression system is a powerful method to produce disulfide-rich peptides, the overexpression of which could be enhanced noticeably through optimization strategies, making it more cost-effective. Since the presence of the His-tag on rMgTx only mildly altered the block equilibrium and binding kinetics, recombinant toxins could be used in ion channel research without removing the tag and could thus reduce the cost and time demand for toxin production.

ASIC1a is required for neuronal activation via low-intensity ultrasound stimulation in mouse brain

Accumulating evidence has shown transcranial low-intensity ultrasound can be potentially a non-invasive neural modulation tool to treat brain diseases. However, the underlying mechanism remains elusive and the majority of studies on animal models applying rather high-intensity ultrasound that cannot be safely used in humans. Here, we showed low-intensity ultrasound was able to activate neurons in the mouse brain and repeated ultrasound stimulation resulted in adult neurogenesis in specific brain regions. In vitro calcium imaging studies showed that a specific ultrasound stimulation mode, which combined with both ultrasound-induced pressure and acoustic streaming mechanotransduction, is required to activate cultured cortical neurons. ASIC1a and cytoskeletal proteins were involved in the low-intensity ultrasound-mediated mechanotransduction and cultured neuron activation, which was inhibited by ASIC1a blockade and cytoskeleton-modified agents. In contrast, the inhibition of mechanical-sensitive channels involved in bilayer-model mechanotransduction like Piezo or TRP proteins did not repress the ultrasound-mediated neuronal activation as efficiently. The ASIC1a-mediated ultrasound effects in mouse brain such as immediate response of ERK phosphorylation and DCX marked neurogenesis were statistically significantly compromised by ASIC1a gene deletion. Collated data suggest that ASIC1a is the molecular determinant involved in the mechano-signaling of low-intensity ultrasound that modulates neural activation in mouse brain.

The biology and evolution of spider venoms

Spiders are diverse, predatory arthropods that have inhabited Earth for around 400 million years. They are well known for their complex venom systems that are used to overpower their prey. Spider venoms contain many proteins and peptides with highly specific and potent activities suitable for biomedical or agrochemical applications, but the key role of venoms as an evolutionary innovation is often overlooked, even though this has enabled spiders to emerge as one of the most successful animal lineages. In this review, we discuss these neglected biological aspects of spider venoms. We focus on the morphology of spider venom systems, their major components, biochemical and chemical plasticity, as well as ecological and evolutionary trends. We argue that the effectiveness of spider venoms is due to their unprecedented complexity, with diverse components working synergistically to increase the overall potency. The analysis of spider venoms is difficult to standardize because they are dynamic systems, fine-tuned and modified by factors such as sex, life-history stage and biological role. Finally, we summarize the mechanisms that drive spider venom evolution and highlight the need for genome-based studies to reconstruct the evolutionary history and physiological networks of spider venom compounds with more certainty.

A Spider Toxin Exemplifies the Promises and Pitfalls of Cell-Free Protein Production for Venom Biodiscovery

Arthropod venoms offer a promising resource for the discovery of novel bioactive peptides and proteins, but the limited size of most species translates into minuscule venom yields. Bioactivity studies based on traditional fractionation are therefore challenging, so alternative strategies are needed. Cell-free synthesis based on synthetic gene fragments is one of the most promising emerging technologies, theoretically allowing the rapid, laboratory-scale production of specific venom components, but this approach has yet to be applied in venom biodiscovery. Here, we tested the ability of three commercially available cell-free protein expression systems to produce venom components from small arthropods, using U₂-sicaritoxin-Sdo1a from the six-eyed sand spider Hexophtalma dolichocephala as a case study. We found that only one of the systems was able to produce an active product in low amounts, as demonstrated by SDS-PAGE, mass spectrometry, and bioactivity screening on murine neuroblasts. We discuss our findings in relation to the promises and limitations of cell-free synthesis for venom biodiscovery programs in smaller invertebrates.

Inhibition of ASIC1a-Mediated ERS Improves the Activation of HSCs and Copper Transport Under Copper Load

Hepatolenticular degeneration (HLD) is an autosomal recessive genetic disease caused by the toxic accumulation of copper in the liver. Excessive copper will disrupt the redox balance in cells and tissues, causing ischemia, hypoxia, and inflammation. Acid-sensitive ion channel 1a is a cationic channel activated by extracellular acid and allowing Ca²⁺ and Na⁺ to flow into cells. Its expression appears in inflammation, arthritis, fibrotic tissue, and damaged environment, but its role in hepatolenticular degeneration has not been studied. This study established a Wistar rat model of high copper accumulation and used CuSO₄ to induce the activation of HSC-T6 in an in vitro experiment. In vivo, Wistar rats were examined to determine the serum copper concentration, serum ALT and AST activities, and liver copper accumulation, and liver tissue HE staining and immunohistochemical analyses were conducted. The expression of ASIC1a, α-SMA, Collagen-Ι, GRP78, XBP1, ATP7B, and CCS were detected. Besides, immunofluorescence technology can detect the expression of the phosphorylated protein in vitro. It is suggested that ASIC1a is involved in the quality control of the endoplasmic reticulum, which degrades mutant ATP7B and increases the accumulation of copper. After blocking or silencing the expression of ASIC1a, ELISA can detect the level of inflammatory factors, the expression of endoplasmic reticulum stress-related factors, and ATP7B was improved in a higher copper environment reduction of copper deposition was observed in liver Timm’s staining. Collectively, we conclude that ASIC1a is involved in the HSC activation induced by copper accumulation and promotes the occurrence of hepatolenticular fibrosis.

New Insectotoxin from Tibellus Oblongus Spider Venom Presents Novel Adaptation of ICK Fold

The Tibellus oblongus spider is an active predator that does not spin webs and remains poorly investigated in terms of venom composition. Here, we present a new toxin, named Tbo-IT2, predicted by cDNA analysis of venom glands transcriptome. The presence of Tbo-IT2 in the venom was confirmed by proteomic analyses using the LC-MS and MS/MS techniques. The distinctive features of Tbo-IT2 are the low similarity of primary structure with known animal toxins and the unusual motif of 10 cysteine residues distribution. Recombinant Tbo-IT2 (rTbo-IT2), produced in E. coli using the thioredoxin fusion protein strategy, was structurally and functionally studied. rTbo-IT2 showed insecticidal activity on larvae of the housefly Musca domestica (LD₁₀₀ 200 μg/g) and no activity on the panel of expressed neuronal receptors and ion channels. The spatial structure of the peptide was determined in a water solution by NMR spectroscopy. The Tbo-IT2 structure is a new example of evolutionary adaptation of a well-known inhibitor cystine knot (ICK) fold to 5 disulfide bonds configuration, which determines additional conformational stability and gives opportunities for insectotoxicity and probably some other interesting features.

A cell-cell interaction format for selection of high-affinity antibodies to membrane proteins

Generating and improving antibodies and peptides that bind specifically to membrane protein targets such as ion channels and G protein-coupled receptors (GPCRs) can be challenging using established selection methods. Current strategies are often limited by difficulties in the presentation of the antigen or the efficiency of the selection process. Here, we report a method for obtaining antibodies specific for whole cell membrane-associated antigens which combines a cell-cell interaction format based on yeast display technology with fluorescence-activated cell sorting of dual fluorescent complexes. Using this method, we were able to direct the affinity maturation of an antagonist antibody specific for the proton-gated ion channel ASIC1a and showed that both the affinity and potency were improved. We were also able to use this method to do kinetic selections to generate clones with better dissociation profiles. In addition, this method was employed successfully to handle the difficult problem of selecting antibodies specific to a GPCR target, the mu-opioid receptor.

Blockade of Acid-Sensing Ion Channels Attenuates Recurrent Hypoglycemia-Induced Potentiation of Ischemic Brain Damage in Treated Diabetic Rats

Diabetes is a chronic metabolic disease and cerebral ischemia is a serious complication of diabetes. Anti-diabetic therapy mitigates this complication but increases the risk of exposure to recurrent hypoglycemia (RH). We showed previously that RH exposure increases ischemic brain damage in insulin-treated diabetic (ITD) rats. The present study evaluated the hypothesis that increased intra-ischemic acidosis in RH-exposed ITD rats leads to pronounced post-ischemic hypoperfusion via activation of acid-sensing (proton-gated) ion channels (ASICs). Streptozotocin-diabetic rats treated with insulin were considered ITD rats. ITD rats were exposed to RH for 5 days and were randomized into Psalmotoxin1 (PcTx1, ASIC1a inhibitor), APETx2 (ASIC3 inhibitor), or vehicle groups. Transient global cerebral ischemia was induced overnight after RH. Cerebral blood flow was measured using laser Doppler flowmetry. Ischemic brain injury in hippocampus was evaluated using histopathology. Post-ischemic hypoperfusion in RH-exposed rats was of greater extent than that in control rats. Inhibition of ASICs prevented RH-induced increase in the extent of post-ischemic hypoperfusion and ischemic brain injury. Since ASIC activation-induced store-operated calcium entry (SOCE) plays a role in vascular tone, next we tested if acidosis activates SOCE via activating ASICs in vascular smooth muscle cells (VSMCs). We observed that SOCE in VSMCs at lower pH is ASIC3 dependent. The results show the role of ASIC in post-ischemic hypoperfusion and increased ischemic damage in RH-exposed ITD rats. Understanding the pathways mediating exacerbated ischemic brain injury in RH-exposed ITD rats may help lower diabetic aggravation of ischemic brain damage.

Ligands of Acid-Sensing Ion Channel 1a: Mechanisms of Action and Binding Sites

The proton-gated cationic channels belonging to the ASIC family are widely distributed in the central nervous system of vertebrates and play an important role in several physiological and pathological processes. ASIC1a are most sensitive to acidification of the external medium, which is the reason for the current interest in their function and pharmacology. Recently, the list of ASIC1a ligands has been rapidly expanding. It includes inorganic cations, a large number of synthetic and endogenous small molecules, and peptide toxins. The information on the mechanisms of action and the binding sites of the ligands comes from electrophysiological, mutational and structural studies. In the present review, we attempt to present a systematic view of the complex pattern of interactions between ligands and ASIC1a.

Secreted Cysteine-Rich Repeat Proteins "SCREPs": A Novel Multi-Domain Architecture

Peptide toxins isolated from animal venom secretions have proven to be useful pharmacological tools for probing the structure and function of a number of molecular receptors. Their molecular structures are stabilized by posttranslational formation of multiple disulfide bonds formed between sidechain thiols of cysteine residues, resulting in high thermal and chemical stability. Many of these peptides have been found to be potent modulators of ion channels, making them particularly influential in this field. Recently, several peptide toxins have been described that have an unusual tandem repeat organization, while also eliciting a unique pharmacological response toward ion channels. Most of these are two-domain peptide toxins from spider venoms, such as the double-knot toxin (DkTx), isolated from the Earth Tiger tarantula (Haplopelma schmidti). The unusual pharmacology of DkTx is its high avidity for its receptor (TRPV1), a property that has been attributed to a bivalent mode-of-action. DkTx has subsequently proven a powerful tool for elucidating the structural basis for the function of the TRPV1 channel. Interestingly, all tandem repeat peptides functionally characterized to date share this high avidity to their respective binding targets, suggesting they comprise an unrecognized structural class of peptides with unique structural features that result in a characteristic set of pharmacological properties. In this article, we explore the prevalence of this emerging class of peptides, which we have named Secreted, Cysteine-rich REpeat Peptides, or “SCREPs.” To achieve this, we have employed data mining techniques to extract SCREP-like sequences from the UniProtKB database, yielding approximately sixty thousand candidates. These results indicate that SCREPs exist within a diverse range of species with greatly varying sizes and predicted fold types, and likely include peptides with novel structures and unique modes of action. We present our approach to mining this database for discovery of novel ion-channel modulators and discuss a number of “hits” as promising leads for further investigation. Our database of SCREPs thus constitutes a novel resource for biodiscovery and highlights the value of a data-driven approach to the identification of new bioactive pharmacological tools and therapeutic lead molecules.

Ginsentides: Cysteine and Glycine-rich Peptides from the Ginseng Family with Unusual Disulfide Connectivity

Ginseng, a popular and valuable traditional medicine, has been used for centuries to maintain health and treat disease. Here we report the discovery and characterization of ginsentides, a novel family of cysteine and glycine-rich peptides derived from the three most widely-used ginseng species: Panax ginseng, Panax quinquefolius, and Panax notoginseng. Using proteomic and transcriptomic methods, we identified 14 ginsentides, TP1-TP14 which consist of 31-33 amino acids and whose expression profiles are species- and tissues-dependent. Ginsentides have an eight-cysteine motif typical of the eight-cysteine-hevein-like peptides (8C-HLP) commonly found in medicinal herbs, but lack a chitin-binding domain. Transcriptomic analysis showed that the three-domain biosynthetic precursors of ginsentides differ from known 8C-HLP precursors in architecture and the absence of a C-terminal protein-cargo domain. A database search revealed an additional 50 ginsentide-like precursors from both gymnosperms and angiosperms. Disulfide mapping and structure determination of the ginsentide TP1 revealed a novel disulfide connectivity that differs from the 8C-HLPs. The structure of ginsentide TP1 is highly compact, with the N- and C-termini topologically fixed by disulfide bonds to form a pseudocyclic structure that confers resistance to heat, proteolysis, and acid and serum-mediated degradation. Together, our results expand the chemical space of natural products found in ginseng and highlight the occurrence, distribution, disulfide connectivity, and precursor architectures of cysteine- and glycine-rich ginsentides as a class of novel non-chitin-binding, non-cargo-carrying 8C-HLPs.

Ion Channels in Pulmonary Hypertension: A Therapeutic Interest?

Pulmonary arterial hypertension (PAH) is a multifactorial and severe disease without curative therapies. PAH pathobiology involves altered pulmonary arterial tone, endothelial dysfunction, distal pulmonary vessel remodeling, and inflammation, which could all depend on ion channel activities (K⁺, Ca²⁺, Na⁺ and Cl^-). This review focuses on ion channels in the pulmonary vasculature and discusses their pathophysiological contribution to PAH as well as their therapeutic potential in PAH.

Anticancer, antimicrobial, and analgesic activities of spider venoms

Spider venoms are complex mixtures composed of a variety of compounds, including salts, small organic molecules, peptides, and proteins. But, the venom of a few species is dangerous to humans. High levels of chemical diversity make spider venoms attractive subjects for chemical prospecting. Many spider venom components show potential activity against a wide range of human diseases. However, the development of novel venom-derived therapeutics requires an understanding of their mechanisms of action. This review will highlight the structures, activities and the possible mechanisms of action of spider venoms and their components against cancer, microbial infections, and pain.

Role of ASIC1a in Aβ-induced synaptic alterations in the hippocampus

Acid-sensing ion channels (ASICs) are widely expressed in the mammalian central nervous system where they play a key role in synaptic transmission and in specific forms of memory. On the other hand, ASICs can be persistently active under pathological conditions contributing to neuronal damage in ischemic stroke, brain trauma, epilepsy and Parkinson's disease. However, to date no experimental evidence has linked ASICs to Alzheimer's disease (AD). Aim of the present work was to investigate, in CA1 pyramidal neurons, the possible involvement of ASIC1a in the Aβ-mediated effect on metabotropic glutamate (mGlu) receptor dependent transmission. We found that, in slices pretreated with Aβ, the pharmacological blockade of ASIC1a restored the increased intrinsic excitability following group I mGlu receptor activation. This suggests that, under certain conditions, ASIC1a might further contribute to the Aβ-related depolarizing response. We have recently demonstrated that ASIC1a is also involved long-term depression (LTD) induced either by low-frequency stimulation or by application of the group I mGlu receptor agonist DHPG. Here, we have shown that psalmotoxin-1, a selective blocker of ASIC1a, rescued the DHPG-LTD facilitation associated with genetic and non-genetic models of AD. Overall, these results suggest that a functional coupling between ASIC1a and mGlu receptors occurs and might contribute to the synaptic alterations associated with AD.

BotAF, a new Buthus occitanus tunetanus scorpion toxin, produces potent analgesia in rodents

This work reports the purification of new potent scorpion neuropeptide, named BotAF, by an activity-guided screening approach. BotAF is a 64-residue long-chain peptide that shares very high similarity with the original β-like scorpion toxin group, in which several peptides have been characterized to be anti-nociceptive in rodents. BotAF administration to rodents does not produce any toxicity or motor impairment, including at high doses. In all models investigated, BotAF turned out to be an efficient peptide in abolishing acute and inflammatory (both somatic and visceral) pain in rodents. It performs with high potency compared to standard analgesics tested in the same conditions. The anti-nociceptive activity of BotAF depends on the route of injection: it is inactive when tested by i.c.v. or i.v. routes but gains in potency when pre-injected locally (in the same compartment than the irritant itself) or by i.t. root 40 to 60 min before pain induction, respectively. BotAF is not an AINS-like compound as it fails to reduce inflammatory edema. Also, it does not activate the opioidergic system as its activity is not affected by naloxone. BotAF does also not bind onto RyR and has low activity towards DRG ion channels (particularly TTX sensitive Na⁺ channels) and does not bind onto rat brain synaptosome receptors. In somatic and visceral pain models, BotAF dose-dependently inhibited lumbar spinal cord c-fos/c-jun mRNA up regulation. Altogether, our data favor a spinal or peripheral anti-nociceptive mode of action of BotAF.

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.

Acid-sensing ion channel (ASIC) structure and function: Insights from spider, snake and sea anemone venoms

Acid-sensing ion channels (ASICs) are proton-activated cation channels that are expressed in a variety of neuronal and non-neuronal tissues. As proton-gated channels, they have been implicated in many pathophysiological conditions where pH is perturbed. Venom derived compounds represent the most potent and selective modulators of ASICs described to date, and thus have been invaluable as pharmacological tools to study ASIC structure, function, and biological roles. There are now ten ASIC modulators described from animal venoms, with those from snakes and spiders favouring ASIC1, while the sea anemones preferentially target ASIC3. Some modulators, such as the prototypical ASIC1 modulator PcTx1 have been studied in great detail, while some of the newer members of the club remain largely unstudied. Here we review the current state of knowledge on venom derived ASIC modulators, with a particular focus on their molecular interaction with ASICs, what they have taught us about channel structure, and what they might still reveal about ASIC function and pathophysiological roles. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Optimized expression of the antimicrobial protein Gloverin from Galleria mellonella using stably transformed Drosophila melanogaster S2 cells

Antimicrobial proteins and peptides (AMPs) are valuable as leads in the pharmaceutical industry for the development of novel anti-infective drugs. Here we describe the efficient heterologous expression and basic characterization of a Gloverin-family AMP derived from the greater wax moth Galleria mellonella. Highly productive single-cell clones prepared by limiting dilution achieved a 100% increase in productivity compared to the original polyclonal Drosophila melanogaster S2 cell line. Comprehensive screening for suitable expression conditions using statistical experimental designs revealed that optimal induction was achieved using 600 µM CuSO4 at the mid-exponential growth phase. Under these conditions, 25 mg/L of the AMP was expressed at the 1-L bioreactor scale, with optimal induction and harvest times ensured by dielectric spectroscopy and the online measurement of optical density. Gloverin was purified from the supernatant by immobilized metal ion affinity chromatography followed by dialysis. In growth assays, the purified protein showed specific antimicrobial activity against two different strains of Escherichia coli.

Potent neuroprotection after stroke afforded by a double-knot spider-venom peptide that inhibits acid-sensing ion channel 1a

Stroke is the second-leading cause of death worldwide, yet there are no drugs available to protect the brain from stroke-induced neuronal injury. Acid-sensing ion channel 1a (ASIC1a) is the primary acid sensor in mammalian brain and a key mediator of acidosis-induced neuronal damage following cerebral ischemia. Genetic ablation and selective pharmacologic inhibition of ASIC1a reduces neuronal death following ischemic stroke in rodents. Here, we demonstrate that Hi1a, a disulfide-rich spider venom peptide, is highly neuroprotective in a focal model of ischemic stroke. Nuclear magnetic resonance structural studies reveal that Hi1a comprises two homologous inhibitor cystine knot domains separated by a short, structurally well-defined linker. In contrast with known ASIC1a inhibitors, Hi1a incompletely inhibits ASIC1a activation in a pH-independent and slowly reversible manner. Whole-cell, macropatch, and single-channel electrophysiological recordings indicate that Hi1a binds to and stabilizes the closed state of the channel, thereby impeding the transition into a conducting state. Intracerebroventricular administration to rats of a single small dose of Hi1a (2 ng/kg) up to 8 h after stroke induction by occlusion of the middle cerebral artery markedly reduced infarct size, and this correlated with improved neurological and motor function, as well as with preservation of neuronal architecture. Thus, Hi1a is a powerful pharmacological tool for probing the role of ASIC1a in acid-mediated neuronal injury and various neurological disorders, and a promising lead for the development of therapeutics to protect the brain from ischemic injury.

Acid-sensing ion channel 1a is required for mGlu receptor dependent long-term depression in the hippocampus

Acid-sensing ion channels (ASICs), members of the degenerin/epithelial Na⁺ channel superfamily, are widely distributed in the mammalian nervous system. ASIC1a is highly permeable to Ca²⁺ and are thought to be important in a variety of physiological processes, including synaptic plasticity, learning and memory. To further understand the role of ASIC1a in synaptic transmission and plasticity, we investigated metabotropic glutamate (mGlu) receptor-dependent long-term depression (LTD) in the hippocampus. We found that ASIC1a channels mediate a component of LTD in P30-40 animals, since the ASIC1a selective blocker psalmotoxin-1 (PcTx1) reduced the magnitude of LTD induced by application of the group I mGlu receptor agonist (S)-3,5-Dihydroxyphenylglycine (DHPG) or induced by paired-pulse low frequency stimulation (PP-LFS). Conversely, PcTx1 did not affect LTD in P13-18 animals. We also provide evidence that ASIC1a is involved in group I mGlu receptor-induced increase in action potential firing. However, blockade of ASIC1a did not affect DHPG-induced polyphosphoinositide hydrolysis, suggesting the involvement of some other molecular partners in the functional crosstalk between ASIC1a and group I mGlu receptors. Notably, PcTx1 was able to prevent the increase in GluA1 S845 phosphorylation at the post-synaptic membrane induced by group I mGlu receptor activation. These findings suggest a novel function of ASIC1a channels in the regulation of group I mGlu receptor synaptic plasticity and intrinsic excitability.

Acid-Sensing Ion Channel 1a Contributes to Airway Hyperreactivity in Mice

Neurons innervating the airways contribute to airway hyperreactivity (AHR), a hallmark feature of asthma. Several observations suggested that acid-sensing ion channels (ASICs), neuronal cation channels activated by protons, might contribute to AHR. For example, ASICs are found in vagal sensory neurons that innervate airways, and asthmatic airways can become acidic. Moreover, airway acidification activates ASIC currents and depolarizes neurons innervating airways. We found ASIC1a protein in vagal ganglia neurons, but not airway epithelium or smooth muscle. We induced AHR by sensitizing mice to ovalbumin and found that ASIC1a^-/- mice failed to exhibit AHR despite a robust inflammatory response. Loss of ASIC1a also decreased bronchoalveolar lavage fluid levels of substance P, a sensory neuropeptide secreted from vagal sensory neurons that contributes to AHR. These findings suggest that ASIC1a is an important mediator of AHR and raise the possibility that inhibiting ASIC channels might be beneficial in asthma.

Molecular dynamics and functional studies define a hot spot of crystal contacts essential for PcTx1 inhibition of acid-sensing ion channel 1a

BACKGROUND AND PURPOSE

The spider-venom peptide PcTx1 is the most potent and selective inhibitor of acid-sensing ion channel (ASIC) 1a. It has centrally acting analgesic activity and is neuroprotective in rodent models of ischaemic stroke. Understanding the molecular details of the PcTx1 : ASIC1a interaction should facilitate development of therapeutically useful ASIC1a modulators. Previously, we showed that several key pharmacophore residues of PcTx1 reside in a dynamic β-hairpin loop; conclusions confirmed by recent crystal structures of the complex formed between PcTx1 and chicken ASIC1 (cASIC1). Numerous peptide : channel contacts were observed in these crystal structures, but it remains unclear which of these are functionally important.

EXPERIMENTAL APPROACH

We combined molecular dynamics (MD) simulations of the PcTx1 : cASIC1 complex with mutagenesis of PcTx1 and rat ASIC1a.

KEY RESULTS

Crystal structures of the PcTx1 : cASIC1 complex indicated that 15 PcTx1 residues form a total of 57 pairwise intermolecular contacts (<5 Å) with 32 channel residues. MD simulations, however, suggested that about half of these interactions do not persist in solution. Mutation to alanine of only eight of 15 PcTx1 contact residues substantially altered ASIC1a inhibition by PcTx1. Our data reveal that many of the peptide-channel interactions observed in the PcTx1 : cASIC1 crystal structures are not important for PcTx1 inhibition of rat ASIC1a.

CONCLUSIONS AND IMPLICATIONS

We identified the atomic interactions that are critical for PcTx1 inhibition of ASIC1a. Our data highlight the value of combining structural information, MD and functional experiments to obtain detailed insight into the molecular basis of protein : protein interactions.

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.

Tarantula toxins use common surfaces for interacting with Kv and ASIC ion channels

Tarantula toxins that bind to voltage-sensing domains of voltage-activated ion channels are thought to partition into the membrane and bind to the channel within the bilayer. While no structures of a voltage-sensor toxin bound to a channel have been solved, a structural homolog, psalmotoxin (PcTx1), was recently crystalized in complex with the extracellular domain of an acid sensing ion channel (ASIC). In the present study we use spectroscopic, biophysical and computational approaches to compare membrane interaction properties and channel binding surfaces of PcTx1 with the voltage-sensor toxin guangxitoxin (GxTx-1E). Our results show that both types of tarantula toxins interact with membranes, but that voltage-sensor toxins partition deeper into the bilayer. In addition, our results suggest that tarantula toxins have evolved a similar concave surface for clamping onto α-helices that is effective in aqueous or lipidic physical environments.

Structural interactions of a voltage sensor toxin with lipid membranes

Protein toxins from tarantula venom alter the activity of diverse ion channel proteins, including voltage, stretch, and ligand-activated cation channels. Although tarantula toxins have been shown to partition into membranes, and the membrane is thought to play an important role in their activity, the structural interactions between these toxins and lipid membranes are poorly understood. Here, we use solid-state NMR and neutron diffraction to investigate the interactions between a voltage sensor toxin (VSTx1) and lipid membranes, with the goal of localizing the toxin in the membrane and determining its influence on membrane structure. Our results demonstrate that VSTx1 localizes to the headgroup region of lipid membranes and produces a thinning of the bilayer. The toxin orients such that many basic residues are in the aqueous phase, all three Trp residues adopt interfacial positions, and several hydrophobic residues are within the membrane interior. One remarkable feature of this preferred orientation is that the surface of the toxin that mediates binding to voltage sensors is ideally positioned within the lipid bilayer to favor complex formation between the toxin and the voltage sensor.

Advanced type 1 diabetes is associated with ASIC alterations in mouse lower thoracic dorsal root ganglia neurons

Acid-sensing ion channels (ASICs) from dorsal root ganglia (DRG) neurons are proton sensors during ischemia and inflammation. Little is known about their role in type 1 diabetes (T1D). Our study was focused on ASICs alterations determined by advanced T1D status. Primary neuronal cultures were obtained from lower (T9-T12) thoracic DRG neurons from Balb/c and TCR-HA(+/-)/Ins-HA(+/-) diabetic male mice (16 weeks of age). Patch-clamp recordings indicate a change in the number of small DRG neurons presenting different ASIC-type currents. Multiple molecular sites of ASICs are distinctly affected in T1D, probably due to particular steric constraints for glycans accessibility to the active site: (i) ASIC1 current inactivates faster, while ASIC2 is slower; (ii) PcTx1 partly reverts diabetes effects against ASIC1- and ASIC2-inactivations; (iii) APETx2 maintains unaltered potency against ASIC3 current amplitude, but slows ASIC3 inactivation. Immunofluorescence indicates opposite regulation of different ASIC transcripts while qRT-PCR shows that ASIC mRNA ranking (ASIC2 > ASIC1 > ASIC3) remains unaltered. In conclusion, our study has identified biochemical and biophysical ASIC changes in lower thoracic DRG neurons due to advanced T1D. As hypoalgesia is present in advanced T1D, ASICs alterations might be the cause or the consequence of diabetic insensate neuropathy.

A novel sea anemone peptide that inhibits acid-sensing ion channels

Sea anemones produce ion channels peptide toxins of pharmacological and biomedical interest. However, peptides acting on ligand-gated ion channels, including acid-sensing ion channel (ASIC) toxins, remain poorly explored. PhcrTx1 is the first compound characterized from the sea anemone Phymanthus crucifer, and it constitutes a novel ASIC inhibitor. This peptide was purified by gel filtration, ion-exchange and reversed-phase chromatography followed by biological evaluation on ion channels of isolated rat dorsal root ganglia (DRG) neurons using patch clamp techniques. PhcrTx1 partially inhibited ASIC currents (IC50∼100 nM), and also voltage-gated K(+) currents but the effects on the peak and on the steady state currents were lower than 20% in DRG neurons, at concentrations in the micromolar range. No significant effect was observed on Na(+) voltage-gated currents in DRG neurons. The N-terminal sequencing yielded 32 amino acid residues, with a molecular mass of 3477 Da by mass spectrometry. No sequence identity to other sea anemone peptides was found. Interestingly, the bioinformatic analysis of Cys-pattern and secondary structure arrangement suggested that this peptide presents an Inhibitor Cystine Knot (ICK) scaffold, which has been found in other venomous organisms such as spider, scorpions and cone snails. Our results show that PhcrTx1 represents the first member of a new structural group of sea anemones toxins acting on ASIC and, with much lower potency, on Kv channels. Moreover, this is the first report of an ICK peptide in cnidarians, suggesting that the occurrence of this motif in venomous animals is more ancient than expected.

Recombinant expression and in vitro characterisation of active Huwentoxin-IV

Huwentoxin-IV (HwTx-IV) is a 35-residue neurotoxin peptide with potential application as a novel analgesic. It is a member of the inhibitory cystine knot (ICK) peptide family, characterised by a compact globular structure maintained by three intramolecular disulfide bonds. Here we describe a novel strategy for producing non-tagged, fully folded ICK-toxin in a bacterial system. HwTx-IV was expressed as a cleavable fusion to small ubiquitin-related modifier (SUMO) in the cytoplasm of the SHuffle T7 Express lysY Escherichia coli strain, which allows cytosolic disulfide bond formation. Purification by IMAC with selective elution of monomeric SUMO fusion followed by proteolytic cleavage and polishing chromatographic steps yielded pure homogeneous toxin. Recombinant HwTx-IV is produced with a C-terminal acid, whereas the native peptide is C-terminally amidated. HwTx-IV(acid) inhibited Nav1.7 in a dose dependent manner (IC50 = 463-727 nM). In comparison to HwTx-IV(amide) (IC50 = 11 ± 3 nM), the carboxylate was ~50 fold less potent on Nav1.7, which highlights the impact of the C-terminus. As the amide bond of an additional amino acid may mimic the carboxamide, we expressed the glycine-extended analogue HwTx-IV(G36)(acid) in the SUMO/SHuffle system. The peptide was approximately three fold more potent on Nav1.7 in comparison to HwTx-IV(acid) (IC50 = 190 nM). In conclusion, we have established a novel system for expression and purification of fully folded and active HwTx-IV(acid) in bacteria, which could be applicable to other structurally complex and cysteine rich peptides. Furthermore, we discovered that glycine extension of HwTx-IV(acid) restores some of the potency of the native carboxamide. This finding may also apply to other C-terminally amidated peptides produced recombinantly.

Ion channels as drug targets in central nervous system disorders

Ion channel targeted drugs have always been related with either the central nervous system (CNS), the peripheral nervous system, or the cardiovascular system. Within the CNS, basic indications of drugs are: sleep disorders, anxiety, epilepsy, pain, etc. However, traditional channel blockers have multiple adverse events, mainly due to low specificity of mechanism of action. Lately, novel ion channel subtypes have been discovered, which gives premises to drug discovery process led towards specific channel subtypes. An example is Na(+) channels, whose subtypes 1.3 and 1.7-1.9 are responsible for pain, and 1.1 and 1.2 - for epilepsy. Moreover, new drug candidates have been recognized. This review is focusing on ion channels subtypes, which play a significant role in current drug discovery and development process. The knowledge on channel subtypes has developed rapidly, giving new nomenclatures of ion channels. For example, Ca(2+)s channels are not any more divided to T, L, N, P/Q, and R, but they are described as Ca(v)1.1-Ca(v)3.3, with even newer nomenclature α1A-α1I and α1S. Moreover, new channels such as P2X1-P2X7, as well as TRPA1-TRPV1 have been discovered, giving premises for new types of analgesic drugs.

Venom toxins in the exploration of molecular, physiological and pathophysiological functions of acid-sensing ion channels

Acid-sensing ion channels (ASICs) are voltage-independent proton-gated cation channels that are largely expressed in the nervous system as well as in some non-neuronal tissues. In rodents, six different isoforms (ASIC1a, 1b, 2a, 2b, 3 and 4) can associate into homo- or hetero-trimers to form a functional channel. Specific polypeptide toxins targeting ASIC channels have been isolated from the venoms of spider (PcTx1), sea anemone (APETx2) and snakes (MitTx and mambalgins). They exhibit different and sometimes partially overlapping pharmacological profiles and are usually blockers of ASIC channels, except for MitTx, which is a potent activator. This review focuses on the use of these toxins to explore the structure-function relationships, the physiological and the pathophysiological roles of ASIC channels, illustrating at the same time the therapeutic potential of some of these natural compounds.

Recombinant expression of margatoxin and agitoxin-2 in Pichia pastoris: an efficient method for production of KV1.3 channel blockers

The K(v)1.3 voltage-gated potassium channel regulates membrane potential and calcium signaling in human effector memory T cells that are key mediators of autoimmune diseases such as multiple sclerosis, type 1 diabetes, and rheumatoid arthritis. Thus, subtype-specific K(v)1.3 blockers have potential for treatment of autoimmune diseases. Several K(v)1.3 channel blockers have been characterized from scorpion venom, all of which have an α/β scaffold stabilized by 3-4 intramolecular disulfide bridges. Chemical synthesis is commonly used for producing these disulfide-rich peptides but this approach is time consuming and not cost effective for production of mutants, fusion proteins, fluorescently tagged toxins, or isotopically labelled peptides for NMR studies. Recombinant production of K(v)1.3 blockers in the cytoplasm of E. coli generally necessitates oxidative refolding of the peptides in order to form their native disulfide architecture. An alternative approach that avoids the need for refolding is expression of peptides in the periplasm of E. coli but this often produces low yields. Thus, we developed an efficient Pichia pastoris expression system for production of K(v)1.3 blockers using margatoxin (MgTx) and agitoxin-2 (AgTx2) as prototypic examples. The Pichia system enabled these toxins to be obtained in high yield (12-18 mg/L). NMR experiments revealed that the recombinant toxins adopt their native fold without the need for refolding, and electrophysiological recordings demonstrated that they are almost equipotent with the native toxins in blocking K(V)1.3 (IC(50) values of 201±39 pM and 97 ± 3 pM for recombinant AgTx2 and MgTx, respectively). Furthermore, both recombinant toxins inhibited T-lymphocyte proliferation. A MgTx mutant in which the key pharmacophore residue K28 was mutated to alanine was ineffective at blocking K(V)1.3 and it failed to inhibit T-lymphocyte proliferation. Thus, the approach described here provides an efficient method of producing toxin mutants with a view to engineering K(v)1.3 blockers with therapeutic potential.

Toxin binding reveals two open state structures for one acid-sensing ion channel

Of the three principal conformations of acid-sensing ion channels (ASICs)—closed, open and desensitized—only the atomic structure of the desensitized conformation had been known. Two recent papers report the crystal structure of chicken ASIC1 in complex with the spider toxin psalmotoxin 1, and one of these studies finds that, depending on the pH, channels are in two different open conformations. Compared with the desensitized conformation, toxin binding induces only subtle structural changes in the lower part of the large extracellular domain but a complete rearrangement of the two transmembrane domains (TMDs), suggesting that desensitization gating (the transition from open to desensitized) is mainly associated with conformational rearrangements of the TMDs. Moreover, the study reveals how two different arrangements of the TMDs in the open state give rise to ion pores with different selectivity for monovalent cations.

Structural plasticity and dynamic selectivity of acid-sensing ion channel-spider toxin complexes

Acid-sensing ion channels (ASICs) are voltage-independent, amiloride-sensitive channels involved in diverse physiological processes ranging from nociception to taste. Despite the importance of ASICs in physiology, we know little about the mechanism of channel activation. Here we show that psalmotoxin activates non-selective and Na(+)-selective currents in chicken ASIC1a at pH 7.25 and 5.5, respectively. Crystal structures of ASIC1a-psalmotoxin complexes map the toxin binding site to the extracellular domain and show how toxin binding triggers an expansion of the extracellular vestibule and stabilization of the open channel pore. At pH 7.25 the pore is approximately 10 Å in diameter, whereas at pH 5.5 the pore is largely hydrophobic and elliptical in cross-section with dimensions of approximately 5 by 7 Å, consistent with a barrier mechanism for ion selectivity. These studies define mechanisms for activation of ASICs, illuminate the basis for dynamic ion selectivity and provide the blueprints for new therapeutic agents.

Structure and activity of the acid-sensing ion channels

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