Acid-sensing ion channel

Acid-sensing ion channels (ASICs) are neuronal voltage-insensitive sodium channels activated by extracellular protons permeable to Na⁺. ASIC1 also shows low Ca²⁺ permeability.

Each acid-sensing ion channel is composed of a 500-560 amino acid sequence, which is constructed into a six transmembrane segment—two per subunit (TMD1 and TMD2), a cytoplasmic amino-carboxyl termini, and a large extracellular domain.[3] 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.^[3]

Pore

The mechanics of the pore function are fundamental to the channel's structure. 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 that change in size and shape depending on channel state.^[3]

The two transmembrane domains (TMD1 and TMD2) of each of the three ASIC subunits are responsible for the channel's pore. 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

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" 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.[3] 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.^[3] 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.^[3] These residues form an acidic pocket that express electrostatic potentials that are responsible for pH-dependency in channel activation and modulation.^[7] 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.^[3]

Function

The role of the ASIC is 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.

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.^[8] There are also many protein kinases that regulate ASIC function through phosphorylation. These include 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.^[8]

There are some other factors that can play a role on the regulation of the ASICs. The presence of matured

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.^[6]

In the central nervous system ASIC's are usually found in the

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.^[8]

ASIC1a channels also play a role in protection against seizure activity. Seizures cause increased, uncontrolled neuronal activity in the brain that releases large quantities of acidic vesicles.^[5] 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.^[8]

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.^[10] 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.^[5] 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.^[10] 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.^[10]

Pharmacology

Due to the role of acid sensing ion channels in pain perception and several pathophysiological processes, they have a

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. A decrease in pH may be due to a variety of reasons including tissue inflammation, ischemic stroke, an accumulation of lactic acid due to increased cellular metabolism. Activation of the channel causes increased permeability of sodium ions which depolarizes the cell and induces the firing of an action potential. The resulting action potentials may be modulated through small molecule inhibitors.

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.^[11][10]

A small molecule inhibitor, A-317567, shows more therapeutic potential than amiloride with a higher specificity to ASIC channels and increased potency. Although A-317567 shows little selectivity for the different kinds of ASIC channels, in vivo findings showed that the side-effects seen with 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.^[10]

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

Commonly used non-steroid anti-inflammatory drugs (NSAIDs) have been found to play a role in ASIC inhibition which contributes to pain modulation. The well-known mechanism for NSAID function is their inhibition of prostaglandin synthesis, a major inflammatory compound. However, findings show that NSAIDs ibuprofen and aspirin inhibit ASICs with IC50 values of 350μM and 260μM, respectively. NSAIDs likely inhibit the ASIC current during acute pain, particularly that caused by tissue inflammation, and thus inhibit the signal to pain-sensing neurons.^[10]

By furthering research on the pharmacological potential in ASIC inhibition, patients suffering with chronic pain and various pathologies associated with acidosis may have greater treatment options in the future. Additionally, drug discovery studies of ASICs provide greater knowledge on the function of the channels themselves and their physiological significance.

References