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Acid-sensing ion channel
Acid-sensing ion channels (ASICs) are neuronal voltage-insensitive sodium channels activated by extracellular protons permeable to Na+, however ASIC1 also shows low Ca++ permeability. ASIC proteins are a subfamily of the ENaC/Deg superfamily of ion channels. These genes have splice variants that encode for several isoforms that are marked by a suffix. ASIC2b is non-functional on its own but modulates channel activity when participating in heteromultimers. ASIC4 has no known function.

Structure
In mammals, acid-sensing ion channels (ASIC) are encoded by five genes producing six ASIC protein subunits: ASIC1, ASIC2, ASIC3, ASIC4, and ASIC5. Three of these protein subunits assemble to form the ASIC, which can combine into both homotrimeric and heterotrimeric channels typically found in both the central nervous system and peripheral nervous system. However, the most common ASIC channels are ASIC1a and ASIC1a/2a and ASIC3. ASIC's have also been found to be conserved in other animals such as toadfish, lampreys, sharks, and freshwater aquarium fish. Each acid-sensing ion channel is composed of a 500-560 amino acid sequence, which is constructs into a six transmembrane segment—two per subunit (TMD1 and TMD2), a cytoplasmic amino-carboxyl termini, and a large extracellular domain. The intracellular amino-carboxyl termini domains are vital to the channel’s intracellular protein interactions and modulations, ion permeability, and gating. However, the gating and mechanics of each acid-sensing ion channel is determined by the combination of ASIC subunits that form its structure. The first crystalized structure of an ASIC channel was observed in 2007 from a chicken ASIC1, yet this only illustrated its inactive form. In 2009, the active form of ASIC1 was observed. To paralyze the structure at this conformation, venom toxic peptides like tarantula toxin psalmotoxin 1 (PcTX1), were used to further conclude structural analysis of ASIC1.

Channel opening
Based on experimentation, to induce channel opening, the presence of protons on the amino acid residues of the thumb domain alters the molecular interaction with the amino acid residues on the “finger” domain. This change in the extracellular region transmits the signal to the transmembrane regions, which further extends the extracellular portion of the pore causing channel opening.

Pore
Between the three ASIC1 subunits, a tunnel extends from the top of the extracellular domains to the cytoplasm of the cell. The central tunnel runs directly between the trimeric unit, where it has large constricted areas, noted as vestibules, which change in size and shape depending on channel state. The two transmembrane domains (TMD1 and TMD2) of each of the three ASIC subunits are responsible for the channel’s pore, which opens upon proton presence. TMD2 is primarily involved with lining of the lumen within the pore and inactivation gate of the channel, where as TMD1 holds the protein within the cell’s lipid bilayer. In a inactivated configuration, the pore center is blocked, only opened upon a change in conformation, allowing ion passage through the intracellular vestibule. The mechanics of the pore function is fundamental to the channel’s structure. TMD1 is connected to the β1 strand of the extracellular domain and TMD2 is connected to β12. β1 to β2 and β11 to β12 are flexible loops within the “palm” region of the extracellular domain that conformationally changes upon the channel’s open state. When activated, β11- β12 flexes, widening the vestibule on the extracellular side of ASIC1. With β12 in a new, slanted position, it changes the angle of TMD2 transmembrane regions opening the channel. In-between the TMD2 segments lay a selectivity filter that forms the narrowest part of the pore determining ASIC permissibility to mostly Na+. In ASIC1, nine amino acid residues, three contributed by each ASIC subunit (Gly443, Ala444, Ser445), form the selectivity filter. Nicknamed the “GAS belt”, all three carbonyl oxygens line the pore, producing a negative potential that contributes to the conductance of cations. The specific amino acid residue of aspartate on the extracellular side lumen of TMD2 in ASIC1 has been linked to the channel’s low Ca2+ conductance. Additionally, The n-termini residues of the transmembrane region has also shown selectivity for Na+, for mutations within this region has altered function and of Na+ conductance.

Extracellular region
ASIC's have a large, fist-like extracellular region that consumes most of the proteins structure. Within its “fist-like” structure there is a wrist, palm, finger, knuckle, thumb and β-ball domains. The “palm” is makes up most of the extracellular domain, formed by seven β-sheets, where as the rest of the secondary structural domains are composed of α-helical segments. Distinguished by its specific amino acid configurations, the extracellular region is fundamental to the induction of activation/inactivation along with pH gating. The specific β-sheet loop area between the “palm” and “thumb” domains has shown involvement in the signal transduction from the extracellular domain to the transmembrane regions, resulting in a conformational change of the ASIC to its open state. At this interface resides proton-sensitive amino acid residues, that once activated, opens the channel, expanding the extracellular vestibule from 15.4 angstroms to 23.4 angstroms. However, it remains fairly inconclusive of which particular residues interact with protons to activate the channel. In 2009, studies may have established a relationship between the aromatic residues Tyr72, Pro287, and Trp288 and proton-gating of the ASIC. The presence of other acidic residues: Asp79, Glu420, Glu426, Asp433, Glu80,Glu374, Glu412, Glu417 also implicate the ASIC structure. These residues form an acidic pocket that express electrostatic potentials that are responsible for pH-dependency in channel activation and modulation. This pocket in the extracellular domain acts as a reserve for cations to concentrate to further assist in Na+ influx. Glycosylation is also apparent within the extracellular region, playing an important role in the trafficking the channel to the membrane’s surface as well as establishing the ASIC’s sensitivity to pH levels. Further experimental evidence has indicated that Ca2+ may also play a pivotal role in modulating proton affinity of ASIC gating both within the pore and on the extracellular domain.

Function
The role of the ASIC to sense reduced levels of extracellular pH and result in a response or signal from the neuron. The ligand that binds to the activation site has long been thought to be exclusively protons, however recent studies have shown that ASIC4 and ASIC1 can be activated at normal pH levels, indicating other types of ligand binders. Under increased acidic conditions, a proton binds to the channel in the extracellular region, activating the ion channel to go through conformational change therefore opening transmembrane domain 2 (TMD2). This results in the influx of sodium ions through the lumen of TMD2. All ASICs are specifically permeable to sodium ions. The only variant is ASIC1a which also has a low permeability to calcium ions. The influx of these cations results in membrane depolarization. Voltage-gated Ca2+ channels are activated resulting in an influx of calcium into the cell. This causes depolarization of the neuron and an excitatory response released. In ASIC1a, Ca2+ increase inside the cell is a result of calcium influx directly through the channel.

Once activated the ASIC can go on to trigger multitudes of different effector proteins and signaling molecules to result in different reactions from the cell. Namely, α-Actinin results in heightened pH sensitivity and desensitization recovery. They can also increase current flow density through the channel. There are also many protein kinases that regulate ASIC function through phosphorylation. These are but aren’t limited to protein kinase A (PKA) and protein kinase C (PKC). There are thought to be many more regulators, yet their effects have not been experimentally concluded.

There are some other factors that can play a role on the regulation of the ASICs. The presence of matured N-linked glycans on the surface of the channel is said to allow the channel to preferentially traffic for ASIC1a. This is a result from the increased N-glycosylation sites on ASIC1a and ASIC2a. The high levels of glycerol (known to expedite protein maturation) on ASIC2 surface also aids in the implication that regulation of these channels’ function is reliant on protein maturation. It is also hypothesized that oxidation plays a role in trafficking.

Pharmacology
Acid sensing ion channels (ASIC) are observed to be activated at pH’s under ~6 with variability depending on the type of channel and its location. Activation of the channel causes increased permeability of sodium ions which depolarizes the cell and induces the firing of an action potential.

Amiloride is an example of an ASIC inhibitor, while not considered highly potent due to an IC50 value in the micromolar range, has allowed for studies on ASIC inhibition effects on migraines. During a migraine, cortical spreading depression is observed which causes ion imbalances and the release of charged molecules which may activate ASIC. Testing of amiloride in rodents, showed a decrease in the cortical spreading depression during a migraine. Studies showed that amiloride acts as a competitive inhibitor of the ASIC chapters. The use of amiloride also showed side effects in rodents due to inhibition of sodium/ calcium exchangers. The inhibition of these exchangers disrupts cellular calcium homeostasis and causes high levels of calcium in the cell which explains the reduced neuroprotective efficacy with the use of amiloride. The findings that have come through due to ASIC inhibition by amiloride are promising and support the therapeutic potential. However due to amiloride’s lack of specificity and potency, further drug development on its structure will need to be done before a drug can be released.

A small molecule inhibitor, A-317567, shows more therapeutic potential than amiloride with a higher specificity to ASIC channels and increased potency. A-317567 shows little selectivity for the different kinds of ASIC channels. In vivo findings showed that the side-effects seen in amiloride use are avoided due to A-317567’s specificity for ASIC. Additionally, A-317567 has the ability to maintain inhibition of sustained currents which could be promising specifically in acidosis-mediated chronic conditions.

The most effective and best-known inhibitor of ASICs is PcTX1. PcTX1 specifically inhibits ASICa and has an IC50 value in the nanomolar range- a smaller IC50 than all other known ASIC inhibitors which have been in the micromolar range. In addition, PcTX1 does not inhibit other voltage gated ion channels or ligand-gated channels. The structure of this inhibitor is 40 amino acids linked with disulfide bonds. It was identified as a peptide toxin from the South American tarantula Psalmopoeus Cambridge.

Location
Most ASIC are expressed in the nervous system. ASIC1, ASIC2, ASIC2b, and ASIC4 are commonly expressed in both the central and peripheral nervous system, while ASIC1b and ASIC3 are typically only located in the peripheral.

In the peripheral nervous system, ASICs are located within the cell bodies of postsynaptic membranes and sensory nerve terminals. Additionally, ASICs are typically found in afferent nerve fibers of the skin, muscles, joints, and viscera, where they have been discovered to be associated with pain, taste, and gastrointestinal functions.

In the central nervous system ASIC’s are usually found in the dorsal horn of the spinal cord. ASIC1 is specifically concentrated in the amygdala, illustrating its role in anxious behavior and ASIC3 has been found in the organ of Corti and spiral ganglion illustrating this specific channel’s role in auditory and vision perception.

Physiology
ASICs are potential drug targets for treating a wide variety of conditions linked to both the CNS and PNS. Of particular interest to pain field is the ASIC3 subtype receptor, which is specifically expressed in nociceptors. This subtype exhibits a biphasic current upon proton activation, where the initial inward Na+ current is shortly followed by a sustained cationic current.

Associated pathologies
ASICs play a role in a number of pathological states of the nervous system, specifically with brain ischemia, retinal damage, and seizure activity.

Brain ischemia
When the brain isn’t getting enough oxygen, it must use anaerobic glycolysis to make ATP which increases the amount of lactic acid in the brain. Ischemic stroke induces acidosis through lactic acid build-up by lowering the pH to about 6.0-6.5.

Brain Ischemia
ASIC1a channels specifically open in response to pH 5.0-6.9 and contribute to the pathology of ischemic brain injury because their activation causes a small increase in Ca2+permeability and an inward flow of Ca2+. ASIC1a channels additionally facilitate the activation of voltage-gated Ca2+ channels and NMDA receptor channels upon initial depolarization, contributing to the major increase in intracellular calcium that results in cell death. A possible mechanism of ASIC1a channel-mediated cell death is due to the activation of other channels, leading to elevated Ca2+ which creates signaling pathways for apoptosis and necrosis in the cell. Gene knockout studies as well as ASIC blockades have shown to reduce brain infarct volume by as much as 60%, suggesting ASIC channels play a major role in the development of the pathological states resulting from acidosis and ischemia induced neuronal injury. The effects of both ASIC and NMDA blockades have been studied to determine the roles of both channels in Ca2+ toxicity and assess their respective contributions. The use of blockade for both channels provides greater neuroprotection than using a blockade for just one channel, and the ASIC blockade creates prolonged effectiveness of the NMDA blockade.

Retinal damage
ASICs are important in retinal function and offer protection in response to bright light. The susceptibility of retinal damage is increased after deletion of the ASIC2 gene. Increased apoptosis occurred in response to bright light in an ASIC2 -/- gene compared to wild type retina.

Seizure activity
ASIC1a channels also play a major role in protection against seizure activity. Seizures cause increased, uncontrolled neuronal activity in the brain that releases large quantities of acidic vesicles. ASIC1a channels open in response and have shown to protect against seizures by reducing their progression. Studies researching this phenomenon have found that deleting the ASIC1a gene resulted in amplified seizure activity.